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Adds a new private class which holds all the members for the tab helper
that need to be reset on every page load.
The encapsulating class is always initialized to keep the diff and logic
change to a minimum.

Bug: 1023483
Change-Id: I1c91ce7eb3d65937e35fcbd949c8d81907adc8f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2097035
Commit-Queue: Robert Ogden <robertogden@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748832}