Disable implicit conversions of scoped_refptr to T* on CrOS.

The implicit conversion has already been disabled for Linux builds for
some time now. https://codereview.chromium.org/510323002 explains the
rationale for proceeding incrementally.

BUG=110610

Review URL: https://codereview.chromium.org/572713003

Cr-Commit-Position: refs/heads/master@{#295234}
1 file changed