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

third_party/WebKit/LayoutTests/compositing/contents-opaque/background-color-expected.txt

 {
   "layers": [
     {
       "name": "LayoutView #document",
       "bounds": [800, 600],
       "contentsOpaque": true,
       "drawsContent": true
     },
     {
       "name": "LayoutBlockFlow DIV class='box composited'",
       "position": [8, 8],
       "bounds": [100, 100]
     },
     {
       "name": "LayoutBlockFlow DIV class='box translucent composited'",
       "position": [8, 108],
       "bounds": [100, 100],
       "drawsContent": true,
-      "backgroundColor": "#00FF007F"
+      "backgroundColor": "#00FF0080"
     },
     {
       "name": "LayoutBlockFlow DIV class='box opaque composited'",
       "position": [8, 208],
       "bounds": [100, 100],
       "contentsOpaque": true,
       "drawsContent": true,
       "backgroundColor": "#008000"
     }
   ]
 }