Auto merge of #8728 - dzbarsky:reset_bitmpa, r=dzbarsky

Reset canvas state when changing bitmap dimensions

Ran http://mxr.mozilla.org/servo/source/tests/wpt/web-platform-tests/html/semantics/embedded-content/the-canvas-element/initial.reset.2dstate.html?force=1#88 to confirm that this code is being executed, but the test still fails because the font isn't implemented and the expando doesn't get reset.

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components/script/dom/htmlcanvaselement.rs

@@ -81,7 +81,7 @@ impl HTMLCanvasElement {
         let size = self.get_size();
         if let Some(ref context) = *self.context.borrow() {
             match *context {
-                CanvasContext::Context2d(ref context) => context.recreate(size),
+                CanvasContext::Context2d(ref context) => context.set_bitmap_dimensions(size),
                 CanvasContext::WebGL(ref context) => context.recreate(size),
             }
         }