Partially automated revert of TreeScope::completeURL/baseURL logic

This reverts the work done in r164469, r165025, and r165098 to support
URL-completion at the TreeScope, rather than the Document, level.

TreeScope URL-completion logic (along with a settable ShadowRoot.baseURI
IDL attribute) was thought to be the solution to resource-loading in
custom elements that themselves were loaded from HTML imports. But in
attempting to implement that logic, enough issues were found to suggest
that this is not the right approach at this time.

See https://www.w3.org/Bugs/Public/show_bug.cgi?id=20976 for more discussion
of the trouble with specifying base URLs at the ShadowRoot level.

BUG=311682

Review URL: https://codereview.chromium.org/137853022

git-svn-id: svn://svn.chromium.org/blink/trunk@165255 bbb929c8-8fbe-4397-9dbb-9b2b20218538