Use event path to detect if anchor has been clicked in WebUIs.

This CL fixes an issue where MD History could not open file URLs due to
the way the global anchor click listener for WebUIs worked. This has been
fixed by checking the event path, as is done in iron-location's global
click handler.

BUG=621260
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2184123003
Cr-Commit-Position: refs/heads/master@{#410972}
3 files changed