[Extensions] Remove a data member from ExtensionWebRequestEventRouter.

This is one in a series of CLs that will move data out of the
ExtensionWebRequestEventRouter that is not keyed by BrowserContext.
This will allow for a more straightforward approach when breaking it
up into multiple per-BrowserContext instances.

This class keeps a mapping from request IDs to their signaled state
for request types. This CL moves that data into a separate global
instance.

Bug: 1433136
Change-Id: I33fb213d80f9867c184cf2a1cbc4caa96a6ae794
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4744640
Reviewed-by: Emilia Paz <emiliapaz@chromium.org>
Commit-Queue: David Bertoni <dbertoni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1180524}