We can use FinalizeIncrementalMarking instead since the only caller of
TryFinalizeIdleIncrementalMarking is IdleNotification, for which staying
within idle deadline is not critical.

This also fixes a bug caused by inconsistent code duplication in
finalization condition in the two functions.

BUG=v8:6325,chromium:715457

Review-Url: https://codereview.chromium.org/2851743002
Cr-Commit-Position: refs/heads/master@{#45054}