Introduce Timer class to clean up public attributes on __gCrWeb.findInPage.

This CL encapsulates code that are related with overtime checking into
Timer class. This helps make the code cleaner and safer.

Bug: 228585
Change-Id: Id2d018b90a6976e39504914dd3f354a276b5601d
Reviewed-on: https://chromium-review.googlesource.com/c/1349714
Commit-Queue: Yi Su <mrsuyi@chromium.org>
Reviewed-by: Peter Lee <pkl@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610945}