Move Document::completeURL() and m_baseURL up to TreeScope

No functional change in this patch; this is in preparation for allowing
authors to set ShadowRoot.baseURI.

While adding a KURL member to TreeScope, I've removed the COMPILE_ASSERT
for the size of TreeScope, as it doesn't seem particularly useful or
consistent with other dom code (we don't check the size of Node or Element).

Also renamed the two-arg version of Document::completeURL() to
completeURLWithOverride() to make it clear that callers should be using the
one-arg TreeScope implementation.

R=esprehn@chromium.org
BUG=311682

Review URL: https://codereview.chromium.org/117323006

git-svn-id: svn://svn.chromium.org/blink/trunk@164469 bbb929c8-8fbe-4397-9dbb-9b2b20218538