Qualify the return value of SkImageDecoder::decode

Qualify the return value of SkImageDecoder::decode

Add a new enum to differentiate between a complete decode and a
partial decode (with the third value being failure). Return this
value from SkImageDecoder::onDecode (in all subclasses, plus
SkImageDecoder_empty) and ::decode.

For convenience, if the enum is treated as a boolean, success and
partial success are both considered true.

Note that the static helper functions (DecodeFile etc) still return
true and false (for one thing, this allows us to continue to use
SkImageDecoder::DecodeMemory as an SkPicture::InstallPixelRefProc in
SkPicture::CreateFromStream).

Also correctly report failure in SkASTCImageDecoder::onDecode when
SkTextureCompressor::DecompressBufferFromFormat fails.

BUG=skia:3037
BUG:b/17419670

Committed: https://skia.googlesource.com/skia/+/2a1208017dd676f94a53bbb228197c3978dbdd8a

[scroggo, 2014-10-17 18:30:07]

scroggo@google.com changed reviewers:
+ djsollen@google.com, halcanary@google.com, reed@google.com

[scroggo, 2014-10-17 18:30:07]

Alternate approach to https://codereview.chromium.org/658343003/

[reed1, 2014-10-17 21:07:26]

lgtm

https://codereview.chromium.org/647023006/diff/20001/include/core/SkImageDeco...
File include/core/SkImageDecoder.h (right):

https://codereview.chromium.org/647023006/diff/20001/include/core/SkImageDeco...
include/core/SkImageDecoder.h:232: kFailure        = 0,    //!< Image failed to
decode. bitmap will be
nit: why specify the actual numeric values? we rarely do that for simple enums.

[scroggo, 2014-10-17 21:11:44]

https://codereview.chromium.org/647023006/diff/20001/include/core/SkImageDeco...
File include/core/SkImageDecoder.h (right):

https://codereview.chromium.org/647023006/diff/20001/include/core/SkImageDeco...
include/core/SkImageDecoder.h:232: kFailure        = 0,    //!< Image failed to
decode. bitmap will be
On 2014/10/17 21:07:26, reed1 wrote:
> nit: why specify the actual numeric values? we rarely do that for simple
enums.

I suppose I could be more explicit, but that is how I accomplish the second
sentence in the description of Result, above.

If someone treats a Result as a boolean, the non-zero items will be interpreted
as true.

[scroggo, 2014-10-22 16:22:54]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2014-10-22 16:23:34]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/647023006/40001

[commit-bot: I haz the power, 2014-10-22 16:27:06]

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

[commit-bot: I haz the power, 2014-10-22 16:27:08]

Try jobs failed on following builders:
  Build-Win-VS2013-x86-Debug-Trybot on client.skia.compile
(http://build.chromium.org/p/client.skia.compile/builders/Build-Win-VS2013-x86...)

[scroggo, 2014-10-22 17:30:02]

On 2014/10/22 16:27:08, I haz the power (commit-bot) wrote:
> Try jobs failed on following builders:
>   Build-Win-VS2013-x86-Debug-Trybot on client.skia.compile
>
(http://build.chromium.org/p/client.skia.compile/builders/Build-Win-VS2013-x86...)

Patch set 4 should fix the Windows build.

[scroggo, 2014-10-22 17:30:02]

On 2014/10/22 16:27:08, I haz the power (commit-bot) wrote:
> Try jobs failed on following builders:
>   Build-Win-VS2013-x86-Debug-Trybot on client.skia.compile
>
(http://build.chromium.org/p/client.skia.compile/builders/Build-Win-VS2013-x86...)

Patch set 4 should fix the Windows build.

[scroggo, 2014-10-22 18:57:37]

The CQ bit was checked by scroggo@google.com

[commit-bot: I haz the power, 2014-10-22 18:58:23]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/647023006/120001

[commit-bot: I haz the power, 2014-10-22 19:07:04]

Committed patchset #7 (id:120001) as 2a1208017dd676f94a53bbb228197c3978dbdd8a