Avoid sending empty bitmaps through proxy canvas

Avoid sending empty bitmaps through proxy canvas

Some back ends may crash when asked to draw an empty bitmap, so we
filter it out here.

BUG=skia:3692
R=djsollen@google.com,reed@google.com

Committed: https://skia.googlesource.com/skia/+/2df6fd650cb12af8eeb7884b2819d2bf8ba7a5bf

[tomhudson, 2015-04-09 15:06:39]

PTAL

[djsollen, 2015-04-09 15:13:07]

what backends fail?  I don't think there is anything preventing java apps from
attempting to draw an empty bitmap.

[tomhudson, 2015-04-09 15:16:39]

This is the crash I was seeing last night when I started trying to diagnosing
the bot failures on N7.

DM resources aren't installed.
factory or cmykjpeg GMs attempt to load a bitmap and fail, so abort their
initialization.
onDraw() then tries to draw a bitmap which has never been initialized aside from
the default empty constructor.
We crash deep inside DisplayListRenderer.

In the bug I suggest that we might want to fix HWUI too.

[tomhudson, 2015-04-09 15:30:44]

tomhudson@google.com changed reviewers:
+ reed@google.com

[tomhudson, 2015-04-09 15:31:02]

Moved checks to SkCanvas pre-virtuals.

[djsollen, 2015-04-09 15:43:39]

lgtm pending mike's ok.

[reed1, 2015-04-09 16:01:20]

lgtm

[tomhudson, 2015-04-09 16:01:29]

The CQ bit was checked by tomhudson@google.com

[commit-bot: I haz the power, 2015-04-09 16:01:52]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1072033002/20001

[commit-bot: I haz the power, 2015-04-09 16:20:22]

Committed patchset #2 (id:20001) as
https://skia.googlesource.com/skia/+/2df6fd650cb12af8eeb7884b2819d2bf8ba7a5bf