bindings: Fix HTMLCanvasElement's IDL definition

The default values of HTMLCanvasElement's toDataURL and toBlob
are not correct, plus it's grammatically wrong to assign IDL
null to DOMString.  This patch fixes them.

Change-Id: I628199d5a8f69858cec0d85448e8862021b96ac1
Bug: 1024695
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1921007Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718072}

third_party/blink/renderer/core/html/canvas/html_canvas_element.idl

@@ -24,7 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-// https://html.spec.whatwg.org/multipage/canvas.html#htmlcanvaselement
+// https://html.spec.whatwg.org/C/canvas.html#htmlcanvaselement
 [
     Exposed=Window,
     HTMLConstructor
@@ -36,12 +36,9 @@
     [RaisesException=Setter, CEReactions] attribute unsigned long width;
     [RaisesException=Setter, CEReactions] attribute unsigned long height;
 
-    // Note: The arguments argument is variadic in the spec, but not here as
-    // only one extra argument is actually used.
-    // FIXME: type should not have a default value.
-    [HighEntropy, MeasureAs=CanvasToDataURL, RaisesException] DOMString toDataURL(optional DOMString type = null, optional any arguments);
+    [HighEntropy, MeasureAs=CanvasToDataURL, RaisesException] DOMString toDataURL(optional DOMString type = "image/png", optional any quality);
 
-    [HighEntropy, MeasureAs=CanvasToBlob, RaisesException] void toBlob(BlobCallback _callback, optional DOMString type = null, optional any arguments);
+    [HighEntropy, MeasureAs=CanvasToBlob, RaisesException] void toBlob(BlobCallback _callback, optional DOMString type = "image/png", optional any quality);
 
     [HighEntropy, RuntimeEnabled=CanvasColorManagement, MeasureAs=CanvasConvertToBlob, RaisesException, CallWith=ScriptState] Promise<Blob> convertToBlob(optional ImageEncodeOptions options);
 };