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This CL adds a method to unload a previously loaded
extension.

Bug: b/73383411
Test: None
Change-Id: Id9f801c5c0296ba9748bfbd4417797f7f4eddb6d
Reviewed-on: https://chromium-review.googlesource.com/1070457Reviewed-by: Alex Sakhartchouk <alexst@chromium.org>
Commit-Queue: Randy Rossi <rmrossi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561610}