DevTools: Get rid of suggest box container element

R=apavlov
NOTRY=true

Review URL: https://codereview.chromium.org/251473003

git-svn-id: svn://svn.chromium.org/blink/trunk@172491 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Added previousElement to SelectorCheckingContext.

To find whether a given last matched element should be considered
as a shadow host or not, added previousElement:

- the last compound selector of a given complex selector matched the
  previousElement,

e.g. suppose that we have a complex selector: "div#foo > .bar span".
In the case, if context.element matches "div#foo", context.previousElement
matches ".bar span".

If a given selector doesn't have any combinator, context.previousElement is
null.

If context.previousElement is null or context.previousElement's treeScope
is different from context.element's treeScope, the context.element is obtained
by walking up from its descendant treeScope.

TEST=fast/dom/shadow/content-pseudo-element-with-nested-shadow.html
BUG=360679

Review URL: https://codereview.chromium.org/247043002

git-svn-id: svn://svn.chromium.org/blink/trunk@172492 bbb929c8-8fbe-4397-9dbb-9b2b20218538