Use dictionaries for context creation attributes. Eliminate custom bindings.

Source/core/html/canvas/Canvas2DContextAttributes.idl

Index: Source/core/html/canvas/Canvas2DContextAttributes.idl
diff --git a/Source/core/html/canvas/Canvas2DContextAttributes.idl b/Source/core/html/canvas/Canvas2DContextAttributes.idl
index f7dfab1fa373c4fbd805806a854dcd74b71a0644..4de5f9bd5e9a4fafd0bc9c2451c0f4ca2b791544 100644
--- a/Source/core/html/canvas/Canvas2DContextAttributes.idl
+++ b/Source/core/html/canvas/Canvas2DContextAttributes.idl
@@ -24,10 +24,9 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-[
-    NoInterfaceObject,
-    WillBeGarbageCollected,
-] interface Canvas2DContextAttributes {
-    attribute boolean alpha;
-    [RuntimeEnabled=ExperimentalCanvasFeatures] attribute DOMString storage;
+dictionary Canvas2DContextAttributes {
+    boolean alpha = true;
+    // Note: the bindings generator ignores the RuntimeEnabled
+    // extended attribute for dictionary members.
+    [RuntimeEnabled=ExperimentalCanvasFeatures] DOMString storage;
 };