Ensure ImageBuffer is never used with ImageBitmapRenderingContext

Ensure ImageBuffer is never used with ImageBitmapRenderingContext

When an HTMLCanvasElement has an ImageBitmapRenderingContext,
calling buffer() will return a buffer that is blank, which is
not the desired result, and triggers the lazy creation of an
persistent buffer that was never used.  This change adds a
DCHECK to catch improper uses, such ase the use case addressed by
https://codereview.chromium.org/2267653005/

BUG=639841
Committed: https://crrev.com/08fd30c773f566dc38208615abb801911808e215
Cr-Commit-Position: refs/heads/master@{#413614}

[Justin Novosad, 2016-08-22 20:52:00]

junov@chromium.org changed reviewers:
+ senorblanco@chromium.org

[Justin Novosad, 2016-08-22 20:52:00]

PTAL

[Stephen White, 2016-08-22 21:23:38]

LGTM

[Justin Novosad, 2016-08-22 21:23:54]

The CQ bit was checked by junov@chromium.org

[commit-bot: I haz the power, 2016-08-22 21:25:43]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-08-23 01:07:51]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-08-23 01:10:09]

Description was changed from

==========
Ensure ImageBuffer is never used with ImageBitmapRenderingContext

When an HTMLCanvasElement has an ImageBitmapRenderingContext,
calling buffer() will return a buffer that is blank, which is
not the desired result, and triggers the lazy creation of an
persistent buffer that was never used.  This change adds a
DCHECK to catch improper uses, such ase the use case addressed by
https://codereview.chromium.org/2267653005/

BUG=639841
==========

to

==========
Ensure ImageBuffer is never used with ImageBitmapRenderingContext

When an HTMLCanvasElement has an ImageBitmapRenderingContext,
calling buffer() will return a buffer that is blank, which is
not the desired result, and triggers the lazy creation of an
persistent buffer that was never used.  This change adds a
DCHECK to catch improper uses, such ase the use case addressed by
https://codereview.chromium.org/2267653005/

BUG=639841

Committed: https://crrev.com/08fd30c773f566dc38208615abb801911808e215
Cr-Commit-Position: refs/heads/master@{#413614}
==========

[commit-bot: I haz the power, 2016-08-23 01:10:10]

Patchset 1 (id:??) landed as
https://crrev.com/08fd30c773f566dc38208615abb801911808e215
Cr-Commit-Position: refs/heads/master@{#413614}