PartitionAlloc: store the root pointer in the super page metadata area.

It enables an additional validity check, and enables further simplifications
later in the patch series such as removing the debug-only pointer from bucket
to root.

BUG=327095

Review URL: https://codereview.chromium.org/129583002

git-svn-id: svn://svn.chromium.org/blink/trunk@164736 bbb929c8-8fbe-4397-9dbb-9b2b20218538
[Custom Elements] Makes HTMLImports suspends following elements upgrade.

This change entangle HTMLImports and Custom Elments so that in-progress
imports block following custom elements from getting upgrade.
It ensures these custom elemnts to get proper definition, which can be
given in preceding imports.

In this CL,

- we introduce CustomElementPendingImport class, which is now an anotehr type of
  CustomElementBaseElementQueue item. The PendingImport has its own
  base element queue. It means that the queue now bocomes the nested list,
  which captures the tree structure of the import tree.
  The item processing of the nested queue is done through process() virtual function.

- Item consumption of the base element queue becomes suspendable.
  If the queue contains any PendingImport, the processing will suspend.
  PendingImport will be removed from the queue once the import is fully loaded.

- HTMLImport had a bug that confuses the notion of "blocking" and "loading"
  the mixture is fixed in this change as well.

R=dominicc
TEST=imports-block-upgrade-in-import.html,importblock-upgrade-in-master.html
BUG=313384

Review URL: https://codereview.chromium.org/121023003

git-svn-id: svn://svn.chromium.org/blink/trunk@164737 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use StringBuilder::append instead of String::append

BUG=268281

Review URL: https://codereview.chromium.org/129673002

git-svn-id: svn://svn.chromium.org/blink/trunk@164738 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Update bindings classes to use OVERRIDE / FINAL when needed

Update bindings classes to use OVERRIDE / FINAL when needed. Those are useful for
error checking or performance when the compiler supports it.

R=esprehn, abarth, haraken
BUG=None

Review URL: https://codereview.chromium.org/128973003

git-svn-id: svn://svn.chromium.org/blink/trunk@164739 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add a CustomSchemeHandler runtime flag to remove ENABLE(CUSTOM_SCHEME_HANDLER).

As a step to remove #ifdef, this cl add a new runtime flag(CustomSchemeHandler), then it removes #if ENABLE(CUSTOM_SCHEME_HANDLER).

BUG=N/A

Review URL: https://codereview.chromium.org/22859043

git-svn-id: svn://svn.chromium.org/blink/trunk@164740 bbb929c8-8fbe-4397-9dbb-9b2b20218538