[Sync::USS] Fix crash when EstimateMemoryUsage in Bookmarks processor

BookmarkModelTypeProcessor::EstimateMemoryUsage()
should check bookmark_tracker_ for nullness before estimating the
memory usage.

The reason is if EstimateMemoryUsage() is called while the model is
being loaded, the bookmark_tracker_ wouldn't have been initialized yet.

Bug: 914745
Change-Id: I93f6ee0972022d4f72dcfdee5261f5efe51c5239
Reviewed-on: https://chromium-review.googlesource.com/c/1375124
Reviewed-by: Mikel Astiz <mastiz@chromium.org>
Commit-Queue: Mohamed Amir Yosef <mamir@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616307}
1 file changed