Clipboard: Change sequence_number from uint64_t to UnguessableToken

The sequence number is supposed to uniquely identify clipboard state,
but in practice this is not true since there can exist multiple
clipboard instances.

Also includes a small change to TestRunnerBindings::CopyImageThen
which may help with some flaky tests.

Bug: 1226356, 1098369
Change-Id: I416a003b3d78966404924a973dfb4890615f8ea4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3011783
Reviewed-by: Darwin Huang <huangdarwin@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Peter Kvitek <kvitekp@chromium.org>
Reviewed-by: Victor Costan <pwnall@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Reviewed-by: Gary Kacmarcik <garykac@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Peter Beverloo <peter@chromium.org>
Commit-Queue: Austin Sullivan <asully@chromium.org>
Cr-Commit-Position: refs/heads/master@{#903688}