REGRESSION: Dropping on file input doesn't work if files are dopped on button text.

This is a regression by r152700 or r153212. DragController assumed a shadow
button in input[type=file] didn't have any child nodes.

* asFileInput: Support a case that the specified node is a Text node in a
  button. Simplify the implementation.

* elementUnderMouse: Support a case that innerNode is a Text node child of a
  ShadowRoot parent.

BUG=280617
TEST=automated

Review URL: https://chromiumcodereview.appspot.com/23538005

git-svn-id: svn://svn.chromium.org/blink/trunk@156952 bbb929c8-8fbe-4397-9dbb-9b2b20218538