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Adds a syncable extension preference for pinned extensions to keep them
in sync. This is used in place of the previous "visible extensions"
paradigm.

Notably extensions are not pinned by default, whereas before they were
initially visible on the toolbar.

Bug: chromium:959932
Change-Id: I23fa0d5314317c079841540e18449646f4da9076
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1598818
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#665682}