Breaks a cyclic dependency in an upcoming CL of mine.

Removing the include of single_thread_task_runner.h here breaks IWYU
all over the place!

The trickiest IWYU thing being exposed by this is that in order for
scoped_refptr<Foo> to be instantiated (not even used), Foo needs to be
fully defined.

This means that, by IWYU, anyone taking a scoped_refptr<Foo> by value or
returning one by value needs to #include "foo.h".

It's still fine to fwd-decl foo when passing via const scoped_refptr<Foo>&
or having a scoped_refptr<Foo> foo_; member in a class constructed/destroyed
out-of-line.

BUG=653916
CQ_INCLUDE_TRYBOTS=master.tryserver.blink:linux_precise_blink_rel
TBR=reviewers for IWUU side-effects (and minor other touchups):
  kmarshall@: blimp/
  danakj@: cc/
           storage/
  bartfab@: chrome/browser/chromeos/policy/
  gene@: chrome/service/
  maxbogue@: components/browser_sync/
             components/sync/
  bengr@: components/data_reduction_proxy/
  fukino@: components/drive/
  dimich@: components/gcm_driver/
  bradnelson@: components/nacl/
  achuith@: components/pairing/
  rsesek@: components/upload_list/
  mef@: components/wifi/
  michaeln@: content/browser/cache_storage/
             content/child/fileapi/
  dgozman@: content/browser/devtools/
  csharrison@: content/browser/loader/
  hbos@: content/renderer/media/
  mkwst@: content/shell/
  pfeldman@: device/usb/
  kbr@: gpu/
  droger@: ios/
  rockot@: ipc/
  dalecurtis@: media/
  agl@: net/
  garykac@: remoting/
  erg@: services/ui/
  dglazkov@: third_party/WebKit/Source/platform/

Review-Url: https://codereview.chromium.org/2443103003
Cr-Commit-Position: refs/heads/master@{#429451}