auto merge of #3732 : mukilan/servo/xhr-issue-3630, r=jdm

This fixes issue #3630
A short summary of the changes:
* Use atomic generation id to cancel inflight requests
* Handles nested calls to abort, open, send inside handlers
* Adds XHRReleaseMsg to delay freeing XHR object till all
  inflight events are processed
* Handles both timeout, errors and abort/open in a symmetric fashion
  i.e All inflight events will be cancelled for timeouts, aborts,
    errors and on calling open.
* Change the ErroredMsg enum to be more symmetric with the returned
  Error enum

I noticed a few possible changes that could make the code for fetch task simpler:
* We can remove the additional timer task and let the fetch task manage 
  its own timer (or maybe the resource loader can do this.)
* The CORS related steps could also be moved into the resource loader.
* Right now upload events are not support. This requires some support 
  from resource loader.

tests/wpt/metadata/XMLHttpRequest/data-uri.htm.ini

@@ -1,33 +1,32 @@
 [data-uri.htm]
   type: testharness
-  expected: TIMEOUT
   [XHR method GET with charset text/plain]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method GET with charset text/plain (base64)]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method GET with charset text/html]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method GET with charset image/png]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method POST with charset text/plain]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method PUT with charset text/plain]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method DELETE with charset text/plain]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method HEAD with charset text/plain]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method UNICORN with charset text/plain]
-    expected: TIMEOUT
+    expected: FAIL
 
   [XHR method GET with charset text/html;charset=UTF-8]
-    expected: TIMEOUT
+    expected: FAIL
 

tests/wpt/metadata/XMLHttpRequest/send-network-error-async-events.sub.htm.ini

@@ -1,6 +0,0 @@
-[send-network-error-async-events.sub.htm]
-  type: testharness
-  expected: TIMEOUT
-  [XmlHttpRequest: The send() method: Fire a progress event named error when Network error happens (synchronous flag is unset)]
-    expected: TIMEOUT
-

tests/wpt/metadata/XMLHttpRequest/send-network-error-sync-events.sub.htm.ini

@@ -1,5 +0,0 @@
-[send-network-error-sync-events.sub.htm]
-  type: testharness
-  [XmlHttpRequest: The send() method: Throw a "throw an "NetworkError" exception when Network error happens (synchronous flag is set)]
-    expected: FAIL
-

tests/wpt/metadata/XMLHttpRequest/xmlhttprequest-timeout-abortedonmain.html.ini

@@ -1,3 +1,3 @@
 [xmlhttprequest-timeout-abortedonmain.html]
   type: testharness
-  expected: CRASH
+  expected: TIMEOUT

tests/wpt/metadata/XMLHttpRequest/xmlhttprequest-timeout-overrides.html.ini

@@ -1,3 +1,3 @@
 [xmlhttprequest-timeout-overrides.html]
   type: testharness
-  expected: CRASH
+  expected: TIMEOUT

tests/wpt/metadata/XMLHttpRequest/xmlhttprequest-timeout-overridesexpires.html.ini

@@ -1,3 +1,3 @@
 [xmlhttprequest-timeout-overridesexpires.html]
   type: testharness
-  expected: CRASH
+  expected: TIMEOUT

tests/wpt/metadata/XMLHttpRequest/xmlhttprequest-timeout-worker-aborted.html.ini

@@ -1,3 +0,0 @@
-[xmlhttprequest-timeout-worker-aborted.html]
-  type: testharness
-  expected: TIMEOUT

tests/wpt/metadata/XMLHttpRequest/xmlhttprequest-timeout-worker-overrides.html.ini

@@ -1,3 +0,0 @@
-[xmlhttprequest-timeout-worker-overrides.html]
-  type: testharness
-  expected: TIMEOUT

tests/wpt/metadata/XMLHttpRequest/xmlhttprequest-timeout-worker-overridesexpires.html.ini

@@ -1,3 +1,6 @@
 [xmlhttprequest-timeout-worker-overridesexpires.html]
   type: testharness
-  expected: TIMEOUT
+
+  [Timeout test: timeout set to expired value before load fires, original timeout at 1000, reset at 400 to 300]
+    disabled: racy test
+

tests/wpt/metadata/XMLHttpRequest/xmlhttprequest-timeout-worker-simple.html.ini

@@ -1,3 +0,0 @@
-[xmlhttprequest-timeout-worker-simple.html]
-  type: testharness
-  expected: TIMEOUT

tests/wpt/metadata/XMLHttpRequest/xmlhttprequest-timeout-worker-twice.html.ini

@@ -1,4 +0,0 @@
-[xmlhttprequest-timeout-worker-twice.html]
-  type: testharness
-  disabled: xhr issue #3630
-  expected: TIMEOUT