Bug 1440961: Make canvas style-related code interact a bit better with Shadow DOM. r=smaug

The only relevant spec reference unfortunately is for color parsing[1], which
mentions:

> Otherwise, set color to the computed value of the 'color' property of element.

Using IsInComposedDoc there should be fine, since the computed style of an
element is well defined there.

The rest look like implementation limitations, for which should not be an issue
to extend the check for the composed doc.

[1]: https://html.spec.whatwg.org/#parsed-as-a-css-color-value

MozReview-Commit-ID: ADnVPNqf3X1

--HG--
extra : rebase_source : ea72f695afe86820d2d04c2b8eca1d17bc356a71

dom/canvas/CanvasRenderingContext2D.cpp

@@ -1226,7 +1226,7 @@ CanvasRenderingContext2D::ParseColor(const nsAString& aString,
     // otherwise resolve it
     nsCOMPtr<nsIPresShell> presShell = GetPresShell();
     RefPtr<nsStyleContext> parentContext;
-    if (mCanvasElement && mCanvasElement->IsInUncomposedDoc()) {
+    if (mCanvasElement && mCanvasElement->IsInComposedDoc()) {
       // Inherit from the canvas element.
       parentContext =
         nsComputedDOMStyle::GetStyleContext(mCanvasElement, nullptr);
@@ -2002,7 +2002,7 @@ CanvasRenderingContext2D::ClearTarget()
 
   SetInitialState();
 
-  if (!mCanvasElement || !mCanvasElement->IsInUncomposedDoc()) {
+  if (!mCanvasElement || !mCanvasElement->IsInComposedDoc()) {
     return;
   }
 
@@ -2739,7 +2739,7 @@ GetFontParentStyleContext(Element* aElement,
                           nsIPresShell* aPresShell,
                           ErrorResult& aError)
 {
-  if (aElement && aElement->IsInUncomposedDoc()) {
+  if (aElement && aElement->IsInComposedDoc()) {
     // Inherit from the canvas element.
     RefPtr<nsStyleContext> result =
       nsComputedDOMStyle::GetStyleContext(aElement, nullptr);
@@ -2919,7 +2919,7 @@ GetFontStyleForServo(Element* aElement, const nsAString& aFont,
   RefPtr<nsStyleContext> parentStyle;
   // have to get a parent style context for inherit-like relative
   // values (2em, bolder, etc.)
-  if (aElement && aElement->IsInUncomposedDoc()) {
+  if (aElement && aElement->IsInComposedDoc()) {
     parentStyle = nsComputedDOMStyle::GetStyleContext(aElement, nullptr);
   } else {
     RefPtr<RawServoDeclarationBlock> declarations =
@@ -4576,7 +4576,7 @@ CanvasRenderingContext2D::DrawOrMeasureText(const nsAString& aRawText,
   bool isRTL = false;
 
   RefPtr<nsStyleContext> canvasStyle;
-  if (mCanvasElement && mCanvasElement->IsInUncomposedDoc()) {
+  if (mCanvasElement && mCanvasElement->IsInComposedDoc()) {
     // try to find the closest context
     canvasStyle =
       nsComputedDOMStyle::GetStyleContext(mCanvasElement, nullptr);