Cleanup old resources faster

These resources are mostly scratch space. As per crbug.com/899940:

- most of this is scratch space textures for intermediate results cached
  to avoid GPU memory churn

- with the UI using OOP-R we don't go idle as often (e.g. cursor blink,
  throbber)

- 15 seconds is a long time to keep it around (many of our regression
  tests don't run this long)

Bug: 899940
Change-Id: Idc077c98551978afa97b18be3dd61784f8898806
Reviewed-on: https://chromium-review.googlesource.com/c/1320790Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608579}

gpu/command_buffer/service/gr_cache_controller.cc

@@ -29,7 +29,7 @@ void GrCacheController::ScheduleGrContextCleanup() {
   if (!purge_gr_cache_cb_.IsCancelled())
     return;
 
-  constexpr int kOldResourceCleanupDelaySeconds = 15;
+  constexpr int kOldResourceCleanupDelaySeconds = 5;
   // Here we ask GrContext to free any resources that haven't been used in
   // a long while even if it is under budget. Below we set a call back to
   // purge all possible GrContext resources if the context itself is not being