Add libjpeg source as third_party/externals dependency

Add libjpeg source as third_party/externals dependency
Committed: https://code.google.com/p/skia/source/detail?r=3334

[epoger, 2012-03-07 17:08:07]

There is also a property change on third_party/externals that the codereview
tool does not show...
___________________________________________________________________
Modified: svn:externals
   - http://gyp.googlecode.com/svn/trunk@1139 gyp

   + http://gyp.googlecode.com/svn/trunk@1139 gyp
http://src.chromium.org/svn/trunk/src/third_party/libjpeg@125399 libjpeg

[epoger, 2012-03-07 17:15:38]

Some notes and potential issues:

1. I have not made gyp changes to actually BUILD libjpeg as part of any of our
existing targets.  For now, I have just imported the libjpeg source code.

2. The libjpeg source we import is maintained within the chromium source tree.

3. I have pinned the dependency at the current chromium tip-of-tree.  If/when
chromium modifies its libjpeg source, the libjpeg source that chromium builds
against will be different than the libjpeg source that Skia builds against.

4. When chromium imports the Skia code, it will not in turn import libjpeg or
other third_party code that Skia has as an external dependency.  I guess when
Skia is built as part of chromium it will be built against chromium's libjpeg
source, but when Skia is built on its own it will be built against the pinned
rev imported into third_party/externals.

I'm sure this is confusing, Rob; we will discuss it live.

[DerekS, 2012-03-07 21:03:30]

LGTM