Add gpu::MailboxHolder to hold state for a gpu::Mailbox

gpu::Mailbox by itself can hold only a texture id, but in common usage it comes
with texture target and syncpoint information for cross-context sharing.  To
reduce repetition of this pattern, gpu::MailboxHolder holds:

* a gpu::Mailbox
* a GL texture target
* a syncpoint index

Refactor other classes to use a gpu::MailboxHolder instead of separate
gpu::Mailbox and associated state.

Syncpoints are created with uint32 indices; make sure all uses of syncpoints
use the appropriate type.

BUG=None
TEST=local build, unittests on CrOS snow, desktop Linux
TBR=piman@chromium.org, enn@chromium.orge, cevans@chromium.org, scherkus@chromium.org

Review URL: https://codereview.chromium.org/132233041

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248612 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248612
Update WebNode::getElementsByTagName() callers to use a WebNodeCollection

Update WebNode::getElementsByTagName() callers to use a WebNodeCollection
instead of a WebNodeList. The WebNode::getElementsByTagName() public API was
updated in Blink r166263 to return a WebNodeCollection instead of a
WebNodeList.

Callers need to be updated so that we can get rid of the current workaround
allowing a WebNodeList to be implicitly converted into a WebNodeCollection.

BUG=235008

Review URL: https://codereview.chromium.org/152133004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248613 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248613