Move compositing code into core/rendering/compositing

This CL moves the compositing-related code from core/rendering into a
compositing subdirectory. These file are quite large an complicated. After
moving them to a directory, the next step is to break down the code into
smaller, easier-to-understand pieces.

R=vollick@chromium.org

Review URL: https://codereview.chromium.org/178353003

git-svn-id: svn://svn.chromium.org/blink/trunk@167697 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Clone template contents in importNode

The template element has no children but contains content, the old importNode behavior only does classical child traversal so special case template elements to import its contents in addition to the normal traversal, like HTMLTemplateElement cloneNode implementation does. Refactor the children traversal importNode into a helper method for better readability and reuse.

BUG=341672

Review URL: https://codereview.chromium.org/173303004

git-svn-id: svn://svn.chromium.org/blink/trunk@167698 bbb929c8-8fbe-4397-9dbb-9b2b20218538