GitLab

Now the navigation cancellation logic should conform to what has been described
in the "PlzNavigate: Navigation cancellation" design document [1]. Added some
new tests to exercise the cancellation cases described in the document.

Note: This is being developed on top of http://crrev.com/912833002.

[1] https://docs.google.com/a/chromium.org/document/d/1lO7_fgppFTDd8PSQfIb888z7vODcw2Sc8r7h5Rbiwcg/edit#

BUG=376014

Review URL: https://codereview.chromium.org/914223002

Cr-Commit-Position: refs/heads/master@{#317083}