Escape 'gomadir' for GN.

No real functionality change (except for people who have " or $ in their gomadir,
but since that didn't work up until this change nobody does that).

R=brettw@chromium.org

Review URL: https://codereview.chromium.org/140123003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245799 0039d316-1c4b-4281-b951-d872f2087c98
Support -Goutput_dir=blahblah in GN-GYP hybrid mode

This supports specifying the output_dir generator flag in the gn-generating-gyp
mode. The value of the output_dir flag is mapped to a gyp variable called gyp_output_dir
and a GN argument by the same name. References from gyp to gn-generated gyp files must
use this variable and references from BUILD.gn files must use the gn argument.

BUG=335760
TBR=brettw for build/config/BUILDCONFIG.gn (looked at in person)

Review URL: https://codereview.chromium.org/142223002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@245800 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 245799
SVN changes up to revision 245800