[HTML Imports] Implement "load" and "error" events.

[HTML Imports] Implement "load" and "error" events.

To implement event delivery, this change does

- Generalize HTMLLinkElement and LinkResource classes so that
  LinkImport class can reap the same plumblings as stylesheet link,
- Introduces HTMLImportsLoaderClient, which is impleented by
  LinkImport, so that HTMLImportLoader notfies the readiness to
  its client. HTMLImportsLoader is capable to multiple client
  for import sharing. In such a case, multiple LinkImport objects
  point same loader object.

In addition, this change refactors HTMLImportsController::createLoader
where instance creation and reuse is now hidden in this method
and callers no longer need to care about that.

BUG=240592
R=dglazkov
TEST=import-onload.html,import-onerror.html

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=155818

[Hajime Morrita, 2013-08-08 10:37:42]

Hi Dimitri, could you take a look?

[dglazkov, 2013-08-08 17:04:18]

lgtm

https://codereview.chromium.org/22573005/diff/1/Source/core/html/HTMLImportLo...
File Source/core/html/HTMLImportLoader.cpp (right):

https://codereview.chromium.org/22573005/diff/1/Source/core/html/HTMLImportLo...
Source/core/html/HTMLImportLoader.cpp:89: // Since DocumentWriter::end() let
setState() reenter, we shouldn't refer m_state here.
refer _to_ m_state.

[Hajime Morrita, 2013-08-09 00:37:14]

Thanks for the review!
I'll update the patch and land it.

[commit-bot: I haz the power, 2013-08-09 00:38:27]

CQ is trying da patch. Follow status at
https://chromium-status.appspot.com/cq/morrita@chromium.org/22573005/7001

[commit-bot: I haz the power, 2013-08-09 03:47:08]

Change committed as 155818