[TaskScheduler] Migrate removable_storage_provider.* to task runner.

Bug: 689520
Change-Id: I7bf48d8f6e63090f627d924656364b92e1b45c6c
Reviewed-on: https://chromium-review.googlesource.com/602997
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Commit-Queue: Istiaque Ahmed <lazyboy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492703}
9 files changed