Fix RGBA alpha parsing and serialization to adhere to W3 standard.

third_party/WebKit/LayoutTests/compositing/draws-content/canvas-background-layer-expected.txt

  {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "LayoutHTMLCanvas CANVAS id='canvas-simple'",
       "position": [13, 8],
       "bounds": [50, 50],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#008000"
     },
     {
       "name": "LayoutHTMLCanvas CANVAS id='canvas-transparent-background'",
       "position": [13, 73],
       "bounds": [50, 50],
       "drawsContent": true,
-      "backgroundColor": "#00FF007F"
+      "backgroundColor": "#00FF0080"
     },
     {
       "name": "LayoutHTMLCanvas CANVAS id='canvas-padding'",
       "position": [13, 138],
       "bounds": [60, 60],
       "drawsContent": true,
       "backgroundColor": "#008000"
     },
     {
       "name": "LayoutHTMLCanvas CANVAS id='canvas-border'",
@@ skipping 13 matching lines @@
       "name": "LayoutHTMLCanvas CANVAS id='canvas-opaque'",
       "position": [13, 333],
       "bounds": [60, 60],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#008000"
     }
   ]
 }