WeakNodeMap should have an ASSERT for node in WeakNodeMap::put()

WeakNodeMap holds trace Ids to corresponding node pointers. Having a null value for the node would be logically incorrect and the ASSERT(!m_nodeToValue.contains(node)); would trigger for the subsequent calls if we allow the NULL values. The callsite should ensure that the values for node pointers are indeed non-NULL.

BUG=390510

Review URL: https://codereview.chromium.org/365633003

git-svn-id: svn://svn.chromium.org/blink/trunk@177370 bbb929c8-8fbe-4397-9dbb-9b2b20218538