Remove the ENABLE_GPU #define and enable_gpu gyp variable

It doesn't make any sense to configure chromium without a gpu and (unsurprisingly)
trying to set this variable to false just results in a broken build.

R=piman

Review URL: https://codereview.chromium.org/25666014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@226964 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 226964