Remove -fstack-protector-strong from the ChromeOS GN build.

This trybots said this was an unknown option, so the comment I was basing my
usage of "strong" on in common.gypi must be out-of-date. The GYP build does not
seem to specify "strong" in any cases except on Mac.

Hook up compiler finding to the Android build.

R=jam@chromium.org

Review URL: https://codereview.chromium.org/99333020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@242746 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 242746