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The bug was as follows:
- mechanism binds to NavigationHandle, storing its NavigationId
- main frame does a subsequent same-document navigation
- PMTabHelper observes this navigation, and updates the internally
  stored NavigationId.
- PM dispatches policy message to mechanism to terminate throttles.
- Policy message looks up WCProxy::LastNavigationId (taken from
  PMTabHelper), and feeds that into the mechanism.
- This doesn't match the original NavId stored in the mechanism, and
  the policy message is dispatched.

Fix:

- Have PMTabHelper/WCProxy independently track same-doc and new-doc
  navigation IDs. Use the last new-doc navigation ID with the policy
  message.

BUG=1084555

Change-Id: I726ace3e06ca83cff99b0dec18372647789910c2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2218228Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Sigurður Ásgeirsson <siggi@chromium.org>
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#772318}