The regression the bug tracks (see the bug link) appears to
be due to identical gap moves in the predecessors of a block
not being moved to the common successor. This CR fixes one
reason that is happening.

BUG=chromium:549262
LOG=n

Review URL: https://codereview.chromium.org/1523393003

Cr-Commit-Position: refs/heads/master@{#32874}