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User expectations on Android is that if the browser goes into the
background and is killed by the OS, and the user switches back to
the app state is restored (Chrome does this now). This is true even
of incognito data.

To limit the potential of leaking private information when incognito
this patch adds encryption of the tab/navigation stack. Chrome on
Android does this as well.

The key used for encryption is randomly generated and stored by the
system. This way, if the system doesn't provide the data (because
the system has determined the app shouldn't restore state), WebLayer
won't be able to decrypt the data.

This means restoring state of incognito data has a different lifetime
then non-incognito data, but that is by design.

Again, this logic matches that of Chrome on Android.

BUG=1033924
TEST=weblayer_support_instrumentation_test_apk
     BrowserFragmentLifecycleTest.restoresPreviousSession

Change-Id: Iaeb721ae942efa1db19ee8366d1cfbf30747cd43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2018482
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#735995}