With this CL, whenever a thread is terminated, its ThreadData is added
to a retired list. When a thread which doesn't have a ThreadData yet
needs one, we first check if a ThreadData from the retired list could be
reused. A ThreadData can be reused if it was previously associated with
a thread that had the same name as the current thread, ignoring trailing
digits. If no ThreadData can be reused, a new one is allocated.

This change is important because TaskScheduler tears down its named
threads when they aren't used for a while and recreates them when they
are needed. We don't want this behavior to increase the number of
ThreadData instances in the process indefinitely over time.

BUG=645196
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2488073002
Cr-Commit-Position: refs/heads/master@{#436745}