Reland "Simplify FrameLoader::Clear(), remove FrameLoader::check_timer"

Clear() is full of strange, old logic. Several clauses are outright unnecessary, or have better-encapsulated homes.

check_timer_ appears to have two uses, one of which is vestigial, the other of which can be better-situated to not require a timer.

BUG=

Review-Url: https://codereview.chromium.org/2878553002
Cr-Commit-Position: refs/heads/master@{#471526}
4 files changed