The mCounter field became exposed as unused after it was initialied in
the constructor initializer list. Previously this was hidden because
it was initialized in the body of the constructor.

BUG=angleproject:1327

Change-Id: I18d9aa8a377d234f4c161ace845edfb9fc7f226c
Reviewed-on: https://chromium-review.googlesource.com/331385Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>