IDL compiler: Handle self-referential interfaces

Minor note: self-referential interfaces already include a header for
themselves (in .cpp), and thus shouldn't include an additional one
in the general header list.
1-line fix and 1-line test case.

BUG=239771
R=haraken
NOTRY=true

Review URL: https://codereview.chromium.org/127443002

git-svn-id: svn://svn.chromium.org/blink/trunk@164672 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Update createElementNS layout test to match WebKit.

While porting a prior CL to WebKit, a layout test added by it got
expanded. This change brings the modifications back to Blink.

Prior CL: https://codereview.chromium.org/122083002/
WebKit bug: https://bugs.webkit.org/show_bug.cgi?id=126553

BUG=330946

Review URL: https://codereview.chromium.org/127473002

git-svn-id: svn://svn.chromium.org/blink/trunk@164673 bbb929c8-8fbe-4397-9dbb-9b2b20218538