MD History/Bookmarks cleanup:  Migrate from var to let/const.

Bug: 792774
Change-Id: If460242199dfe9e7cead62def00d3cb6e861eaa3
Reviewed-on: https://chromium-review.googlesource.com/855588
Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
Reviewed-by: calamity <calamity@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528116}
34 files changed