WebLayer: automatically reload gone renderers when tab becomes visible

Test:
1. run weblayer shell
2. press home button to hide app
3. kill the renderer process `adb shell ps` and `adb shell kill`
4. switch back to weblayer shell

Automated tests to be added in a follow up.

Bug: 1146058
Change-Id: I6164f8ca377416437ff09beee6afa37fa4f36f9a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2521338


Commit-Queue: Evan Stade <estade@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824881}