Remove reference to FILE thread in thread_restrictions.h

R=robliao@chromium.org

Bug: 689520
Change-Id: Id5e138389b16c02a612d000007c2447cd1e2701d
Reviewed-on: https://chromium-review.googlesource.com/580527Reviewed-by: Robert Liao <robliao@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488811}

base/threading/thread_restrictions.h

@@ -119,7 +119,8 @@ class ThreadTestHelper;
 // 1) If a thread should not be allowed to make IO calls, mark it:
 //      base::ThreadRestrictions::SetIOAllowed(false);
 //    By default, threads *are* allowed to make IO calls.
-//    In Chrome browser code, IO calls should be proxied to the File thread.
+//    In Chrome browser code, IO calls should be proxied to a TaskRunner with
+//    the base::MayBlock() trait.
 //
 // 2) If a function makes a call that will go out to disk, check whether the
 //    current thread is allowed: