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This is a reland of f5d2f7eb

ThumbnailImage's data can be null. This adds a null check to
GetCompressedDataSizeInBytes() in which case it returns 0. This reland
also adds a unit test for this case.

Original change's description:
> Add metrics for thumbnail memory usage
>
> Adds two metrics logged every 5 minutes:
> * Per-thumbnail compressed data memory usage
> * Total compressed data memory usage
>
> The former has a sample for each thumbnail every 5 minutes, while the
> latter has exactly one every 5 minutes.
>
> Bug: 11261624
> Change-Id: I1595c6a6b771f73e3be9a5497d1cf6f6ec3d1af7
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462685
> Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
> Reviewed-by: Dana Fried <dfried@chromium.org>
> Commit-Queue: Collin Baker <collinbaker@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#815714}

Bug: 11261624
Change-Id: I330f6a74041faf7ee1758e8b5f7a146456161f8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2463510
Commit-Queue: Collin Baker <collinbaker@chromium.org>
Auto-Submit: Collin Baker <collinbaker@chromium.org>
Reviewed-by: Dana Fried <dfried@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816820}