Add missing ScopedAllowBlocking to TabManagerDelegateChromeos.

TabManagerDelegateChromeos reads the "low memory margin" and "available"
memory values from kernel-provided entries in the filesystem, when
determining how many processes need to be discarded to ease memory
pressure. This causes AssertIOAllowed to fail if discards ever trigger
in a build with DCHECKs enabled.

In principle the reads from these kernel-provided files should not block
so add a ScopedAllowBlocking to work-around the issue.

Bug: 
Change-Id: I472f7fca8a4780e6a6abceb0a3baff541c8e3a9b
Reviewed-on: https://chromium-review.googlesource.com/738224
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Cheng-Yu Lee <cylee@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#511951}

• base/threading/thread_restrictions.h[diff]
• chrome/browser/resource_coordinator/tab_manager_delegate_chromeos.cc[diff]