Make FrameTree::uniqueName() more unique.

Currently, it only guarantees no two siblings have the same unique name. This
patch tries much harder to guarantee that no two frames in the same page have
the same unique name.

BUG=352651
TEST=http/tests/history/frameset-repeated-name.html

Review URL: https://codereview.chromium.org/207143002

git-svn-id: svn://svn.chromium.org/blink/trunk@169764 bbb929c8-8fbe-4397-9dbb-9b2b20218538