Remove unused imports

In preparation for renaming PausableObject remove unused imports.

BUG=907125

Change-Id: Id6cdc2561d1df04313ac6f16edea81d0b45445f3
Reviewed-on: https://chromium-review.googlesource.com/c/1407650
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622457}
7 files changed