Make textInputFlags use static AtomicStrings

This method is called a lot during browser launch. Instead of
reconstructing the same AtomicString every time (without even
ConstructFromLiteral) pass in a local static AtomicString.

Review URL: https://codereview.chromium.org/554933003

git-svn-id: svn://svn.chromium.org/blink/trunk@181916 bbb929c8-8fbe-4397-9dbb-9b2b20218538