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This CL calls WillStartRequest on the current NavigationHandle when a
navigation is starting. This mimics the SimulateRedirect call, and enables
NavigationThrottles which override WillStartRequest to be unit testable
using the content unit test framework.

Note: like SimulateRedirect, there is no current support for the throttles
actually cancelling / deferring the navigation.

BUG=627501

Review-Url: https://codereview.chromium.org/2157153003
Cr-Commit-Position: refs/heads/master@{#407150}