Use 'HashTraits::PeekInType' in the ListHashSet 'find', 'contains' and 'remove' methods

Use the 'HashTraits::PeekInType' as an argument type
in ListHashSet 'find', 'contains' and 'remove' methods
(same way it is done now in HashMap and HashSet) thus
enabling possible optimizations (e.g. 'RefPtrValuePeeker'
for 'RefPtr' argument type).

The patch also contains a test proving that now there is
no unneeded argument de/refing at 'ListHashSet<RefPtr>::contains' call.

Using of 'HashTraits::PeekInType' in the ListHashSet 'add'
methods requires some extra changes and will be enabled
with a following patch.

Review URL: https://codereview.chromium.org/190753003

git-svn-id: svn://svn.chromium.org/blink/trunk@169084 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Implement /content/ combinator.

spec: shadow styling
http://dev.w3.org/csswg/shadow-styling/#content-combinator

The added layout tests are basically the same as ::content's, because /content/ combinator should have almost the same behavior (except css text, ::content with descendant combinator).

BUG=349334
TEST=fast/dom/shadow/cascade-of-treeboundary-crossing-rules.html,fast/dom/shadow/content-combinator-css-text.html,fast/dom/shadow/content-combinator-dynamic-attribute-change.html,fast/dom/shadow/content-combinator-in-inert-shadow.html,fast/dom/shadow/content-combinator-scoped.html,fast/dom/shadow/content-combinator-with-cat.html,fast/dom/shadow/content-combinator-with-hat.html,fast/dom/shadow/content-combinator-with-host-pseudo-class-2.html,fast/dom/shadow/content-combinator-with-host-pseudo-class.html,fast/dom/shadow/content-combinator.html,fast/dom/shadow/multiple-content-combinator.html,fast/dom/shadow/no-style-sharing-with-uncommon-attribute-and-content-combinator.html

Review URL: https://codereview.chromium.org/187353003

git-svn-id: svn://svn.chromium.org/blink/trunk@169085 bbb929c8-8fbe-4397-9dbb-9b2b20218538