Clean up gyp settings for libjpeg

The gyp settings for libjpeg currently have a number of problems ...
- Settings for using the system libjpeg are replicated in the gyp files for both libjpeg and libjpeg_turbo.
- The default setting for use_system_libjpeg is different depending on whether libjpeg or libjpeg_turbo is in use. This is unintuitive, as libjpeg_turbo is a drop-in replacement for libjpeg.

This patch improves the situation ...
- Improve documentation about the interaction between use_system_libjpeg and use_libjpeg_turbo.
- Use the libjpeg gyp file whenever use_system_libjpeg==1. This means that the settings for using the system libjpeg are only needed here. (A later patch will remove them from libjpeg_turbo's gyp file). It also means that the libjpeg_turbo repository can be excluded when use_system_libjpeg==1.
- Move all settings concerned with use_system_libjpeg to the libjpeg gyp file.

Note that with this change, we lose the OS-specific defaults for use_system_libjpeg when use_libjpeg_turbo==0. These defaults are currently not used, as use_libjpeg_turbo==1 always.

Review URL: https://chromiumcodereview.appspot.com/10696082

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@147254 0039d316-1c4b-4281-b951-d872f2087c98

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