MIPS: Resolve chromium and android webview build conflicts.

The mips related gyp variables and compiler flags for webview
and chromium builds are set externally. The CL ensures this
behaviour and prevents conflicts in configuration with standalone builds.

TEST=
BUG=
R=jkummerow@chromium.org, paul.lind@imgtec.com

Review URL: https://codereview.chromium.org/707883002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25243 ce2b1a6d-e550-0410-aec6-3dcde31c8c00