Remove the |max_threads| argument from CreateAndSetSimpleTaskScheduler().

We want to change TaskSchedulerImpl so that it always has 4 thread pools. This
is incompatible with a CreateAndSetSimpleTaskScheduler() function that takes
a global maximum number of threads as argument. Therefore, this CL removes
that argument. Since existing callers didn't have comments to justify how the
value of this argument was chosen, they shouldn't care too much about this
change.

This CL also adds a |name| argument to CreateAndSetSimpleTaskScheduler()
that is used to label threads and histograms associated with the created
TaskScheduler.

BUG=690706
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_cronet_tester

Review-Url: https://codereview.chromium.org/2729523006
Cr-Commit-Position: refs/heads/master@{#458450}
14 files changed