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Implement safebrowsing shutdown when the browser shuts down
(i.e. when last instance of BrowserImpl is destroyed).

This ensures proper cleanup of the safebrowsing service,
pending requests and the corresponding remote safe browsing
database manager.

In particular in this patch:
- move safe_browsing_service ownership to browser_process
- shutdown safebrowsing on destruction of last BrowserImpl
- add code to count live BrowserImpl instances (corresponds
  to BrowserImpl.Observer on the java side)
- add checks and make sure to stop the safebrowsing
  database manager on IO thread.

BUG=1071533,1015418

Change-Id: I39ad06a489df20454f2970db287b1f11b81ded1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2132448
Commit-Queue: Tim Volodine <timvolodine@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#762004}