Fix flags for ARM GN build.

I accidentally used underscores which gives undefined option errors.

TBR=scottmg

Review URL: https://codereview.chromium.org/135733005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@244838 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 244838