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Currently the storage data is send to and from the web ui using int,
however when the size gets over 2GB, it will overflow and crash the
browser. This CL convert the data into double and adds unit test for
large storage data.

BUG=960726,943735

Change-Id: I085410fdd3da1674a0b44d1a445927b015219f47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1602234Reviewed-by: Joshua Bell <jsbell@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Maggie Cai <mxcai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658359}