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In preparation for CBCM reporting unit tests, this extends the mock
infrastructure for policy objects that will be involved. In particular:

- A new mock BrowserStatePolicyConnector was added
- The TestChromeBrowserState builder now supports building the browser
  state with a mock BrowserStatePolicyConnector
- The TestChromeBrowserStateManager can return a TestChromeBrowserState
  by path
- The TestingApplicationContext now creates a real
  BrowserPolicyConnector if its accessor is called

These are all needed for the ProfileReportGeneratorIOS unit tests, which
will come in a separate CL.

Bug: 1055318
Change-Id: I3428e56d695c014d63245be6493d527787343cce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2495265Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Guillaume Jenkins <gujen@google.com>
Cr-Commit-Position: refs/heads/master@{#820786}