Add support for URL.searchParams getter.

Add the missing piece to our URLSearchParams implementation;
the readonly attribute for URL, URL.searchParams:

 https://url.spec.whatwg.org/#dom-url-searchparams

The currently spec'ed connection between URL and URLSearchParams is
a lot less general than previous designs, hence the object lifetime
complexities it ran into (see https://codereview.chromium.org/143313002/)
falls away.

Intent to Implement and Ship (for URLSearchParams and this URL attribute):

 https://groups.google.com/a/chromium.org/d/msg/blink-dev/grHZDbldP04/JdsoQ169AQAJ

R=mkwst
BUG=303152

Review URL: https://codereview.chromium.org/1860623002

Cr-Commit-Position: refs/heads/master@{#386189}
19 files changed