Bug 1362914 - Drop initial values of properties other than font-size and font-family in serialization of canvas font attribute. r=manishearth

According to the canvas spec [1], we must drop initial values other than
font-size and font-family when we serialize canvas font attribute.
As for font-size and font-family, the default values, '10px sans-serif' [2]
are not dropped at all.

[1] https://html.spec.whatwg.org/multipage/scripting.html#dom-context-2d-font
[2] https://html.spec.whatwg.org/multipage/scripting.html#canvastextdrawingstyles

MozReview-Commit-ID: EyOxzJCQYQl

--HG--
extra : rebase_source : d358a8e97fa7849a320a3619e746a526fe2a8439

dom/canvas/CanvasRenderingContext2D.cpp

@@ -2901,9 +2901,7 @@ GetFontStyleForServo(Element* aElement, const nsAString& aFont,
   // parsed (including having line-height removed).  (Older drafts of
   // the spec required font sizes be converted to pixels, but that no
   // longer seems to be required.)
-  Servo_DeclarationBlock_SerializeOneValue(declarations,
-                                           eCSSProperty_font,
-                                           &aOutUsedFont);
+  Servo_SerializeFontValueForCanvas(declarations, &aOutUsedFont);
 
   return sc.forget();
 }

layout/style/ServoBindingList.h

@@ -473,6 +473,11 @@ SERVO_BINDING_FUNC(Servo_StyleSet_GetBaseComputedValuesForElement,
                    const mozilla::ServoElementSnapshotTable* snapshots,
                    mozilla::CSSPseudoElementType pseudo_type)
 
+// For canvas font.
+SERVO_BINDING_FUNC(Servo_SerializeFontValueForCanvas, void,
+                   RawServoDeclarationBlockBorrowed declarations,
+                   nsAString* buffer)
+
 // Style-struct management.
 #define STYLE_STRUCT(name, checkdata_cb)                            \
   struct nsStyle##name;                                             \