HTML Imports: Take care of cycles according to the spec.

HTML Imports: Take care of cycles according to the spec.

This fixes for regression that blocks import cycle indefinitely.
The change has two points:

 * Let isBlockingFollowers() see HTMLImportChild::isFirst(). The
   isFirst() corresponds to the notion of "marked as branch" in the spec.
 * Make CustomElementMicrosaskImportStep aware of tree shuffling done by
   HTMLImportChild::normalize().

TEST=import-cycle-*.html
R=dglazkov@chromium.org, dominicc@chromium.org
BUG=373461
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=174141

[Hajime Morrita, 2014-05-15 21:42:36]

PTAL?

This is embarrassing. My test should've covered this but didn't
because coverage was weak and the check doesn't see the loading completion. In
fact, newly added tests found more problem than reported.

I need to merge this. So I'd like to postpone possible big refactoring later.

[dglazkov, 2014-05-15 22:35:16]

lgtm >_>

[Hajime Morrita, 2014-05-16 08:24:04]

The CQ bit was checked by morrita@chromium.org

[commit-bot: I haz the power, 2014-05-16 08:24:25]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/morrita@chromium.org/289993004/1

[commit-bot: I haz the power, 2014-05-16 08:39:14]

Change committed as 174141