Remove unnecessary image packing code.

Remove unnecessary image packing code.

Basically we only need to unpack images if
1) it's from HTML elements, where the colors are either RGBA8 or BGRA8 (used by skia); or
2) it has color and alpha (code might ask for premultiply or unmultiply alpha)

BUG=497352
TEST=webgl conformance tests
R=kbr@chromium.org,bajones@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=197300

[Zhenyao Mo, 2015-06-17 20:40:59]

bajones, kbr: could you please take a look at your earliest convenience?  I have
quite a large size CL for ES3 formats pending this.

[Zhenyao Mo, 2015-06-17 20:41:30]

Tested with WebGL conformance tests 1.0.4 on Linux, no regression, so these
codes are truly not needed.

[Zhenyao Mo, 2015-06-17 21:04:06]

Not yet, a lot more code can be deleted.

[Zhenyao Mo, 2015-06-17 22:23:59]

My apology, but the CL is good as is.  It turned out the extra code I want to
delete is needed in deed.

Please review.

[bajones, 2015-06-17 22:25:31]

On 2015/06/17 22:23:59, Zhenyao Mo wrote:
> My apology, but the CL is good as is.  It turned out the extra code I want to
> delete is needed in deed.
> 
> Please review.

non-owners LGTM

[Ken Russell (switch to Gerrit), 2015-06-17 23:44:16]

Excellent cleanup. LGTM

[Zhenyao Mo, 2015-06-17 23:44:45]

The CQ bit was checked by zmo@chromium.org

[commit-bot: I haz the power, 2015-06-17 23:45:32]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1181283005/20001

[commit-bot: I haz the power, 2015-06-18 01:14:07]

Committed patchset #2 (id:20001) as
https://src.chromium.org/viewvc/blink?view=rev&revision=197300