Add test cases for selectAll command with shadow DOM

This patch adds a layout test file for executing "selectAll" command on shadow DOM tree for checking regression.

This test script checks:
 - select all from selecting host content => select all document
 - select all from selecting shadow tree => select immediate child of shadow root which contains start position
and behavior in shadow DOM tree in editable host.

BUG=234038, 408887
TEST=n/a; This patch adds test.

Review URL: https://codereview.chromium.org/654173003

git-svn-id: svn://svn.chromium.org/blink/trunk@184043 bbb929c8-8fbe-4397-9dbb-9b2b20218538