SkPDF: detect YUV-JPEG without relying on ImageGenerator

SkPDF:  detect YUV-JPEG without relying on ImageGenerator

JPEG/JFIF References:
*   http://www.w3.org/Graphics/JPEG/itu-t81.pdf
*   http://www.w3.org/Graphics/JPEG/jfif3.pdf

BUG=476721
BUG=446940
Committed: https://skia.googlesource.com/skia/+/96287f7af7ff6aaa48b8d28ec6b7b79836da2d7c

[hal.canary, 2015-05-07 02:57:15]

The CQ bit was checked by halcanary@google.com to run a CQ dry run

[commit-bot: I haz the power, 2015-05-07 02:57:36]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1133443003/1

[commit-bot: I haz the power, 2015-05-07 03:03:35]

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

[commit-bot: I haz the power, 2015-05-07 03:03:37]

Dry run: This issue passed the CQ dry run.

[hal.canary, 2015-05-07 13:56:51]

halcanary@google.com changed reviewers:
+ msarett@google.com

[msarett, 2015-05-07 14:22:23]

lgtm.  I like this code.  I find it far more readable and understandable than
what I've seen in libjpeg.  Having said that, I would feel more comfortable with
this change if we could say with 100% certainty that linking with libjpeg (and
using jpeg_read_header) is not a viable option.

[reed1, 2015-05-07 14:42:22]

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

[reed1, 2015-05-07 14:42:23]

Perhaps we can make this a (private) utility function somewhere near our codecs,
so we can more easily document and test it (independent of PDF)?

[hal.canary, 2015-05-07 16:13:05]

On 2015/05/07 14:22:23, msarett wrote:
> lgtm.  I like this code.  I find it far more readable and understandable than
> what I've seen in libjpeg.  Having said that, I would feel more comfortable
with
> this change if we could say with 100% certainty that linking with libjpeg (and
> using jpeg_read_header) is not a viable option.

I'm having a lot of trouble linking to libjpeg within skia inside chromium. 
Let's land this.

[reed1, 2015-05-07 16:21:07]

We definitely want to learn how to link against libjpeg in chrome for Leon's
extended work. Have you discussed your difficulties w/ Leon?

[hal.canary, 2015-05-07 18:18:37]

The CQ bit was checked by halcanary@google.com to run a CQ dry run

[hal.canary, 2015-05-07 18:18:38]

The patchset sent to the CQ was uploaded after l-g-t-m from msarett@google.com
Link to the patchset: https://codereview.chromium.org/1133443003/#ps60001
(title: " ")

[commit-bot: I haz the power, 2015-05-07 18:19:00]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1133443003/60001

[hal.canary, 2015-05-07 18:21:00]

unit tests and proper grayscale support added.  Any objections to landing it as
it stands, and iterating?

[commit-bot: I haz the power, 2015-05-07 18:36:36]

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

[commit-bot: I haz the power, 2015-05-07 18:36:37]

Dry run: This issue passed the CQ dry run.

[msarett, 2015-05-07 18:42:57]

lgtm

No objections to landing it as it stands and iterating.

[hal.canary, 2015-05-07 18:45:55]

The CQ bit was checked by halcanary@google.com

[commit-bot: I haz the power, 2015-05-07 18:46:15]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1133443003/60001

[commit-bot: I haz the power, 2015-05-07 18:47:05]

Committed patchset #4 (id:60001) as
https://skia.googlesource.com/skia/+/96287f7af7ff6aaa48b8d28ec6b7b79836da2d7c