Add an API to report debugName in GraphicsLayer

Provide an API in GraphicsLayer to report debugName which is useful for
debugging sometime. Now debugName is pushed from GraphicsLayer to
WebLayer, while this patch makes it possible to get debugName on demand.

BUG=269258
TEST=NONE

Review URL: https://chromiumcodereview.appspot.com/18473002

git-svn-id: svn://svn.chromium.org/blink/trunk@156119 bbb929c8-8fbe-4397-9dbb-9b2b20218538