Gut gyp_chromium, common.gypi.

This change removes all of the code referencing gyp_chromium that
isn't needed to keep the closure_compilation builder working and/or
also needed for proper functioning of the gclient hooks.

R=brettw@chromium.org, dbeam@chromium.org
BUG=570091, 632206
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2355673003
Cr-Commit-Position: refs/heads/master@{#420182}