Remove unused inspector fields

This patch removes two unused fields in the inspector:
InspectorDOMAgent: m_client
InspectorTimelineAgent: m_memoryAgent

No tests, no change in behavior.

Review URL: https://codereview.chromium.org/108093004

git-svn-id: svn://svn.chromium.org/blink/trunk@164076 bbb929c8-8fbe-4397-9dbb-9b2b20218538