[Mojo] Split up Mojo's monolithic GYP file

mojo.gyp was getting kind of out of control. This CL splits it up into a few
GYPI files based on the top-level mojo directories.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/97153003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238007 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238007