profile: Integrate the time profiler with the macOS signpost

infrastructure.

With this change, if you supply the `-Z signpost` flag, Instruments.app
can display Servo events overlaid with callstack data. Even better, you
can get call stack profiling for individual Servo events (one layout,
one network request, one style recalculation, etc.)

This adds a dependency on the `signpost` crate. On unsupported OS's,
this crate is an no-op.

etc/patch-trace-template.py

@@ -0,0 +1,108 @@
+#!/usr/bin/env python
+
+# Copyright 2013 The Servo Project Developers. See the COPYRIGHT
+# file at the top-level directory of this distribution.
+#
+# Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+# http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+# <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+# option. This file may not be copied, modified, or distributed
+# except according to those terms.
+
+import re
+import subprocess
+import sys
+from xml.etree import ElementTree
+from xml.etree.ElementTree import Element
+
+if len(sys.argv) < 3:
+    sys.stderr.write("""usage: patch-trace-template.py components/profile_traits/time.rs path/to/my.tracetemplate
+
+`time.rs` is usually located in `components/profile_traits/time.rs`.
+Trace templates are typically located in `~/Library/Application Support/Instruments/Templates`.
+The supplied trace template must contain the "Points of Interest" instrument.
+Output is written to standard output and should typically be piped to a new `.tracetemplate` file.
+
+Example:
+    patch-trace-template.py \\
+    components/profile_traits/time.rs \\
+        ~/Library/Application\\ Support/Instruments/Templates/MyTemplate.tracetemplate > \\
+        ~/Library/Application\\ Support/Instruments/Templates/MyServoTemplate.tracetemplate
+""")
+    sys.exit(0)
+
+rust_source = open(sys.argv[1], 'r')
+lines = iter(rust_source)
+for line in lines:
+    if line.lstrip().startswith("pub enum ProfilerCategory"):
+        break
+
+code_pairs = []
+regex = re.compile(r"\s*(\w+)\s*=\s*([0-9]+|0x[a-fA-F0-9]+),?\s*$")
+for line in lines:
+    if line.lstrip().startswith("}"):
+        break
+
+    match = regex.match(line)
+    if match is None:
+        continue
+    code_pairs.append((match.group(2), match.group(1)))
+
+xml = subprocess.check_output(["plutil", "-convert", "xml1", "-o", "-", sys.argv[2]])
+plist = ElementTree.ElementTree(ElementTree.fromstring(xml))
+
+elems = iter(plist.findall("./dict/*"))
+for elem in elems:
+    if elem.tag != 'key' or elem.text != '$objects':
+        continue
+    array = elems.next()
+    break
+
+elems = iter(array.findall("./*"))
+for elem in elems:
+    if elem.tag != 'string' or elem.text != 'kdebugIntervalRule':
+        continue
+    dictionary = elems.next()
+    break
+
+elems = iter(dictionary.findall("./*"))
+for elem in elems:
+    if elem.tag != 'key' or elem.text != 'NS.objects':
+        continue
+    objects_array = elems.next()
+    break
+
+child_count = sum(1 for _ in iter(array.findall("./*")))
+
+for code_pair in code_pairs:
+    number_index = child_count
+    integer = Element('integer')
+    integer.text = str(int(code_pair[0], 0))
+    array.append(integer)
+    child_count += 1
+
+    string_index = child_count
+    string = Element('string')
+    string.text = code_pair[1]
+    array.append(string)
+    child_count += 1
+
+    dictionary = Element('dict')
+    key = Element('key')
+    key.text = "CF$UID"
+    dictionary.append(key)
+    integer = Element('integer')
+    integer.text = str(number_index)
+    dictionary.append(integer)
+    objects_array.append(dictionary)
+
+    dictionary = Element('dict')
+    key = Element('key')
+    key.text = "CF$UID"
+    dictionary.append(key)
+    integer = Element('integer')
+    integer.text = str(string_index)
+    dictionary.append(integer)
+    objects_array.append(dictionary)
+
+plist.write(sys.stdout, encoding='utf-8', xml_declaration=True)