When navigator.share() is called with files, the files are saved
to a new directory, and sharesheet::SharesheetService is used to
present the user with a choice of share targets.

ShareServiceUnitTest now adds blobs to blob storage context.
Test cases TotalBytes and FileBytes are removed as they
became redundant with
https://chromium-review.googlesource.com/c/chromium/src/+/2415431
and they risk flakiness by creating large blobs.

Spec: https://w3c.github.io/web-share/#share-method

Bug: 1110119
Change-Id: I8c7d44f899cf8c0d31cd87fcb0ce77b25f5a98b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2422107
Commit-Queue: Eric Willigers <ericwilligers@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Reviewed-by: Maggie Cai <mxcai@chromium.org>
Reviewed-by: Melissa Zhang <melzhang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810759}