Fix "unreachable code" warnings (MSVC warning 4702) in Blink.

BUG=346399
TEST=none
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/203743002

git-svn-id: svn://svn.chromium.org/blink/trunk@169581 bbb929c8-8fbe-4397-9dbb-9b2b20218538