HTML Imports: A speculative fix for fast/dom/custom/imports-custom-element-abort.html

The test occasionally crashes by touching null HTMLImportsController:m_master in a timer function.
This change cancel the timer when m_master is cleared, and add an ASSERT() to make sure that
|m_master| is available when timer is fired.

BUG=355640
TEST=imports-custom-element-abort.html
R=dominicc

Review URL: https://codereview.chromium.org/209333012

git-svn-id: svn://svn.chromium.org/blink/trunk@169962 bbb929c8-8fbe-4397-9dbb-9b2b20218538