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When profile is loaded, parse the bookmark tree to find the total count.
Adds and tests new bookmark stat, which saves to a new histogram. The
collection method is run in the profile manager class, after it has been
initialized.

Bug: 245392
Change-Id: I6a72549e2cbc197e92de4edbcf7b362212ff2c39
Reviewed-on: https://chromium-review.googlesource.com/1238901Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Commit-Queue: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#594540}