Support lossy and lossless <canvas>.toDataURL for webp

LayoutTests/fast/canvas/toDataURL-supportedTypes.html

Index: LayoutTests/fast/canvas/toDataURL-supportedTypes.html
diff --git a/LayoutTests/fast/canvas/toDataURL-supportedTypes.html b/LayoutTests/fast/canvas/toDataURL-supportedTypes.html
index 02475d8e8c74eb362bdd6c2b29fc1cec7c183a35..ea1d1d90e286daa6034a7e10989297c2d791df1f 100644
--- a/LayoutTests/fast/canvas/toDataURL-supportedTypes.html
+++ b/LayoutTests/fast/canvas/toDataURL-supportedTypes.html
@@ -25,7 +25,8 @@
         var supportedMIMETypes = [
             "image/png",
             "image/jpeg",
-            "image/gif",

[urvang, 2015/09/01 19:54:08]

Curious: How was "image/gif" working before?

[Noel Gordon, 2015/09/01 23:20:25]

The existing test expectations report:

Given MIMEType: image/gif
Used MIMEType: image/png
MIME types DIFFER.

so no change in behavior with this CL. Unsupported image/ types must be encoded
as PNG per the <canvas> spec.

+            "image/webp",
+            "image/webplossless",
         ];
 
         function testToDataURL()
@@ -41,6 +42,7 @@
             testMIMEType(undefined);
 
             // Test other formats that we don't support
+            testMIMEType("image/gif");
             testMIMEType("image/x-webkitbitmap");
         }