Remove the need for asCoeff in SkXfermode.

Remove the need for asCoeff in SkXfermode.

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/dcfb7cf336cafffd8d149c908b615e1deaa3a49b

[egdaniel, 2015-01-21 16:02:38]

egdaniel@google.com changed reviewers:
+ bsalomon@google.com, reed@google.com

[egdaniel, 2015-01-21 16:02:38]

The main goal of this cl was to remove the need for asCoeff from SkXfermode.
This was originally needed to support the gpu but that has since been removed.

The two other callers were in cpp files outside of SkXfermode that were really
trying to make general queries about the xfer mode (draw opacity and conflating
coverage and alpha), but were doing so by "knowing" too much internal knowledge
of the implementation of the xfer modes. I've abstracted these ideas into
virtuals on SkXfermode so that an individual xfer more can decide how to
implement these queries.

[bsalomon, 2015-01-21 17:51:07]

lgtm... I think there is more cleanup that could be done on the signature and
impl isPaintOpaque, but that's kind of expanding the scope.

https://codereview.chromium.org/864833002/diff/1/src/core/SkPaintPriv.cpp
File src/core/SkPaintPriv.cpp (right):

https://codereview.chromium.org/864833002/diff/1/src/core/SkPaintPriv.cpp#new...
src/core/SkPaintPriv.cpp:24: if (255 == paint->getAlpha() &&
I think we usually use 0xff

[egdaniel, 2015-01-22 14:38:26]

The CQ bit was checked by egdaniel@google.com

[commit-bot: I haz the power, 2015-01-22 14:39:28]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/864833002/20001

[commit-bot: I haz the power, 2015-01-22 14:40:57]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2015-01-22 14:40:58]

Try jobs failed on following builders:
  Build-Ubuntu13.10-Clang-x86_64-Debug-Trybot on client.skia.compile
(http://build.chromium.org/p/client.skia.compile/builders/Build-Ubuntu13.10-Cl...)

[egdaniel, 2015-01-22 14:43:37]

The CQ bit was checked by egdaniel@google.com

[commit-bot: I haz the power, 2015-01-22 14:44:26]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/864833002/40001

[commit-bot: I haz the power, 2015-01-22 14:52:33]

Committed patchset #3 (id:40001) as
https://skia.googlesource.com/skia/+/dcfb7cf336cafffd8d149c908b615e1deaa3a49b