url: work around a gcc warning bug.

Gcc 4.9 has a bug in reporting false positive array out of bound
warning. This cl adds progma to disable the warning.

BUG=chromium:392872
TEST=The warning is gone.

Review URL: https://codereview.chromium.org/384843002

git-svn-id: https://src.chromium.org/chrome/trunk/src/url@282914 4ff67af0-8c30-449e-8e8b-ad334ec8d88c