[blink] Add crash keys to investigate ScripedIdleTaskController OOM.

ScriptedIdleTaskController is a top source of OOM, spread across many
magic signatures (go/chrome-idle-tasks-oom-fdoray). However, it's not
clear if this is mostly caused by a large number of IdleTask objects, a
large number of scheduler idle tasks (there may be more scheduler idle
tasks than IdleTasks) or a problem in in the data structure used by the
scheduler to hold idle tasks. To get to the bottom of this, this CL
adds crash keys with:
- The maximum number of pending IdleTasks observed in a single
  ScriptedIdleTaskController.
- The maximum number of pending scheduler idle tasks observed from
  a single ScriptedIdleTaskController.

Bug: 40940722, 355192731, 324950597, 355226107, 355226107
Change-Id: Id36881a1ee7dcd95d471e17755414fc1c6610f0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5828926


Reviewed-by: Scott Haseley <shaseley@chromium.org>
Commit-Queue: Francois Pierre Doray <fdoray@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Francois Pierre Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1349382}