Followup to r177831.

Extend the nodeToIdMap by calling set() rather than update
the end() iterator.

R=haraken@chromium.org
BUG=340522
NOTRY=true

Review URL: https://codereview.chromium.org/382633008

git-svn-id: svn://svn.chromium.org/blink/trunk@177841 bbb929c8-8fbe-4397-9dbb-9b2b20218538