Support an ALPHA_8 UIResourceBitmap format

cc/resources/resource_format.h

Index: cc/resources/resource_format.h
diff --git a/cc/resources/resource_format.h b/cc/resources/resource_format.h
index 816c28579139744cebec7f1b8760e3371398273f..b51ac7cfe721d968a706419e416595494264f891 100644
--- a/cc/resources/resource_format.h
+++ b/cc/resources/resource_format.h
@@ -15,6 +15,7 @@ enum ResourceFormat {
   RGBA_8888,
   RGBA_4444,
   BGRA_8888,
+  ALPHA_8,

[danakj, 2014/08/13 17:17:27]

May I ask so I can answer this in the future, why do we need both this and
LUMINANCE_8 in here?

[jdduke (slow), 2014/08/13 18:01:57]

Each corresponds to a distinct GL format that has very different semantics.
ALPHA_8 effectively provides a black RGB texture with pixel-varying alpha
values. LUMINANCE_8 provides a grayscale RGB texture with pixel-varying
intensities but a constant alpha of 1.

[jdduke (slow), 2014/08/13 18:07:15]

I suppose it might be able to achieve the same effect with LUMINANCE using the
proper blending function, but our blending mode support currently appears to be
somewhat limited.

[danakj, 2014/08/13 18:46:06]

Which blending functions are you refering to here?

[jdduke (slow), 2014/08/13 18:53:37]

Layer::SetBlendMode(SkXfermode::Mode blend_mode). I tried a few of the supported
modes blindly, but none of them yielded the desired effect (referring to
https://codereview.chromium.org/367173003/). Of note, Android L's overscroll
effect happens to use the kSrcATop_Mode, which  doesn't appear to be supported. 

RGBA8 works just fine, but we were under the impression that the ALPHA_8 format
could save us some memory along the way.

   LUMINANCE_8,
   RGB_565,
   ETC1,