OOPIF: Add frame_unique_name to FrameNavigationEntry.

This holds the unique name of the frame in which the navigation took
place.  This is stable after back/forward and restore, whereas the
FrameTreeNode ID will change over time.

BUG=236848, 502317
TEST=No behavior change yet.

Review URL: https://codereview.chromium.org/1407853005

Cr-Commit-Position: refs/heads/master@{#358746}
9 files changed