Auto merge of #14404 - BorisChiou:Bug1317179, r=Manishearth

Use XPCOM string bindings instead of Gecko_Utf8SliceToString

These are the servo-side changes for [bug 1317179](https://bugzilla.mozilla.org/show_bug.cgi?id=1317179). @Manishearth has already reviewed them there. I'd like to merge these patches until the gecko-side changes for [bug 1317179](https://bugzilla.mozilla.org/show_bug.cgi?id=1317179) is landed.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix [bug 1317179](https://bugzilla.mozilla.org/show_bug.cgi?id=1317179).
- [X] These changes do not require tests because there are existing tests for this in mozilla-central

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components/style/gecko_bindings/bindings.rs

@@ -86,7 +86,6 @@ use gecko_bindings::structs::nsIPrincipal;
 use gecko_bindings::structs::nsIURI;
 use gecko_bindings::structs::nsMainThreadPtrHolder;
 use gecko_bindings::structs::nsRestyleHint;
-use gecko_bindings::structs::nsString;
 use gecko_bindings::structs::nsStyleBackground;
 unsafe impl Send for nsStyleBackground {}
 unsafe impl Sync for nsStyleBackground {}
@@ -488,10 +487,6 @@ extern "C" {
                                               *const ::std::os::raw::c_char,
                                           aLength: u32) -> bool;
 }
-extern "C" {
-    pub fn Gecko_Utf8SliceToString(aString: *mut nsString, aBuffer: *const u8,
-                                   aBufferLen: usize);
-}
 extern "C" {
     pub fn Gecko_FontFamilyList_Clear(aList: *mut FontFamilyList);
 }
@@ -1095,7 +1090,7 @@ extern "C" {
                                                         RawServoDeclarationBlockBorrowed,
                                                     property: *mut nsIAtom,
                                                     is_custom: bool,
-                                                    buffer: *mut nsString);
+                                                    buffer: *mut nsAString_internal);
 }
 extern "C" {
     pub fn Servo_DeclarationBlock_Count(declarations: