Rename AtomicString::string() to AtomicString::getString().

The current name of the getter will collide with WTF::String once Blink
is rewritten to Chromium style. Prefix it with get to avoid this
problem.

BUG=582312
R=haraken@chromium.org

Review URL: https://codereview.chromium.org/1835773002

Cr-Commit-Position: refs/heads/master@{#383460}
64 files changed