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This CL also changes the SharedWorkerService::Observer interface so
that SharedWorkerId replaces SharedWorkerInstance to uniquely
identify shared workers outside of content/.

The motivation behind this change is to make it cheaper to use
the shared worker identifier as a key into a map, since copying
a SharedWorkerInstance is quite expensive.

Bug: 1054596
Change-Id: I7270d9e5fabac2d2e00aecd407a745a1cc779791
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2067389
Commit-Queue: Patrick Monette <pmonette@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Reviewed-by: Matt Falkenhagen <falken@chromium.org>
Reviewed-by: Ahmed Fakhry <afakhry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743988}