ui/gfx: optimize color format conversions by using memcpy in jpeg_codec.cc

ui/gfx: optimize color format conversions by using memcpy in jpeg_codec.cc

This is similar idea which was already applied in png_codec.cc.
https://codereview.chromium.org/397673002/
(Sorry for making another review request.)

BUG=none
R=erg@chromium.org, sky@chromium.org, dcheng@chromium.org
TEST(DONE&PASSED)=gfx_unittests --gtest_filter="JPEGCodec*"

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=284585

[hyunki, 2014-07-18 13:37:03]

Please take a look. :) 
And sorry for making another review request.
Thank you!

[Elliot Glaysher, 2014-07-18 17:13:26]

So do you have measurements that these are better? I'm assuming that you're
relying on the compiler using something like __builtin_memcpy() here. (I would
expect actually performing a function call would be slower.)

[hyunki, 2014-07-18 21:37:22]

I see. I'll measure it and share the results. Thanks.

[hyunki, 2014-07-19 04:03:26]

I did simple performance test for StripAlpha() function.

* Test environments
- pixel_width: 10000000
- on the linux desktop with Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz

* Before(without patch): 48ms (average for 1000 runs)
* After(with patch): 41ms (average for 1000 runs)

In summary, this patch can speed it up by about 15%.

[Elliot Glaysher, 2014-07-21 17:59:08]

lgtm

[hyunki, 2014-07-21 19:25:16]

On 2014/07/21 17:59:08, Elliot Glaysher wrote:
> lgtm

Thanks for the review. :)

[hyunki, 2014-07-21 19:25:29]

The CQ bit was checked by hyunki.baik@samsung.com

[commit-bot: I haz the power, 2014-07-21 19:26:56]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/hyunki.baik@samsung.com/404793004/1

[commit-bot: I haz the power, 2014-07-21 23:45:29]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  android_dbg_triggered_tests on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/android_dbg_triggered...)
  win_chromium_x64_rel on tryserver.chromium
(http://build.chromium.org/p/tryserver.chromium/builders/win_chromium_x64_rel/...)

[commit-bot: I haz the power, 2014-07-22 01:37:10]

Change committed as 284585