DevTools: implement front_end-side Inspect Element mode.

Review URL: https://chromiumcodereview.appspot.com/23506018

git-svn-id: svn://svn.chromium.org/blink/trunk@157087 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Have SVGURIReference API deal with Document references, not pointers

Have SVGURIReference API deal with Document references, not pointers as it
requires the document to be non-null. This makes it clear to the caller
that a non-null Document should be provided. Most caller actually already
have a Document reference so this change avoids having to convert them
into pointers.

BUG=281400

Review URL: https://chromiumcodereview.appspot.com/23685007

git-svn-id: svn://svn.chromium.org/blink/trunk@157088 bbb929c8-8fbe-4397-9dbb-9b2b20218538