Make slider thumb painting bail out when the node is null.

Make slider thumb painting bail out when the node is null.

The other media control painting methods do handle a null node in
toParentMediaElement (as can happen with a pseudo-element, such as
::-webkit-scrollbar). The slider thumb painting should too.

There are currently unfounded assertions that this node is not null,
but author style can make it null, so a proper check is appropriate.

A layout test has been added for this case.

BUG=412154
Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=181656

[jbroman, 2014-09-09 15:13:28]

jbroman@chromium.org changed reviewers:
+ dglazkov@chromium.org, eseidel@chromium.org, tkent@chromium.org

[eseidel, 2014-09-09 15:18:07]

The CQ bit was checked by eseidel@chromium.org

[eseidel, 2014-09-09 15:18:08]

lgtm

[commit-bot: I haz the power, 2014-09-09 15:18:45]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/jbroman@chromium.org/556453007/1

[eseidel, 2014-09-09 15:19:01]

These are specifically "anonymous renderers" any renderer w/o an associated node
is "anonymous".  It's easy to create anonymous renderers, and yes the
RenderTheme code needs to know that they can exist. :)

[commit-bot: I haz the power, 2014-09-09 16:11:14]

Committed patchset #1 (id:1) as 181656