Merge _PageState to _RunState in page_runner.py

Merge _PageState to _RunState in page_runner.py

The motivation behind this merge is to create a single place
that manages the browser & tab state before running multiple
page, before running each page, after running each page, and
after running multiple pages. Next patches will refactor
this _RunState object to SharedPageState as planned in
https://docs.google.com/a/google.com/document/d/1on2ATPE8Y7a0SLcd4cl3nAiSthVpVLU6OoVZatEavEA/edit.

Besides the merging of PageState and RunState, this patch
also renames _RunPage -> _RunPageAndProcessErrorIfNeeded
and _PrepareAndRunPage -> _RunPageAndRetryRunIfNeeded to
clear up the distinction what each of these functions does.

BUG=418278

Review URL: https://codereview.chromium.org/652813002

Cr-Commit-Position: refs/heads/master@{#299425}