Extract tree root responsibility out of HTMLImportsController.

Extract tree root responsibility out of HTMLImportsController.

The HTMLImportsController wishes to become more of an Actor-style object,
and it being a root of the import tree leads to ambiguities in responsibilities.
So we should split these responsibilities cleanly.

R=morrita
BUG=

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=175108

[dglazkov, 2014-05-28 17:28:50]

Depends on https://codereview.chromium.org/303643005/.

[Hajime Morrita, 2014-05-28 20:38:52]

lgtm assuming tests pass. YOU MADE IT!

[dglazkov, 2014-05-28 23:44:42]

The CQ bit was checked by dglazkov@chromium.org

[commit-bot: I haz the power, 2014-05-28 23:45:06]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dglazkov@chromium.org/301803002/1

[commit-bot: I haz the power, 2014-05-29 01:01:51]

FYI, CQ is re-trying this CL (attempt #1).
The failing builders are:
  linux_blink_dbg on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_dbg/builds/1...)
  linux_blink_rel on tryserver.blink
(http://build.chromium.org/p/tryserver.blink/builders/linux_blink_rel/builds/9877)
  linux_gpu on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/linux_gpu/builds/...)

[commit-bot: I haz the power, 2014-05-29 01:25:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2014-05-29 01:25:03]

Try jobs failed on following builders:
  linux_gpu on tryserver.chromium.gpu
(http://build.chromium.org/p/tryserver.chromium.gpu/builders/linux_gpu/builds/...)

[dglazkov, 2014-05-29 16:34:51]

The CQ bit was checked by dglazkov@chromium.org

[commit-bot: I haz the power, 2014-05-29 16:36:59]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/dglazkov@chromium.org/301803002/20001

[commit-bot: I haz the power, 2014-05-30 07:57:51]

Change committed as 175108