Add polymer-selector and polymer-selection to polymer.

These polymer UI components are needed for the PDF extension.

BUG=303491
R=bshe@chromium.org, koz@chromium.org

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=246216

Review URL: https://codereview.chromium.org/141483005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246514 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246514
Add some code to help debug RenderFrameImpl::createChildFrame failures.

BUG=336870

Review URL: https://codereview.chromium.org/140823020

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@246515 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 246515