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Adds a button to the internals page which sends all pending reports
immediately.

This button simply sends all reports for now. A more
complicated UI was considered, which would allow individual rows to
be selected and sent. However, this requires changes to the
ConversionStorage interface, and the existing interface can be
extended if this functionality is desirable.

This change also restructures communication between
ConversionManagerImpl and ConversionReporterImpl to be callback based.
This is done to allow binding the HandleSentReport callback to a
BarrierClosure which holds the WebUI callback. This allows the WebUI
to update once all reports have finished sending.

Bug: 1057240
Change-Id: If82773b01b9df98d6d1f52cf35aee8fd6bdcf751
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2192436
Commit-Queue: John Delaney <johnidel@chromium.org>
Reviewed-by: Ken Buchanan <kenrb@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#768561}