Update help text
Update help text

This change updates the help text to the current controls.

screenshot: https://docs.google.com/file/d/0B8Y78t3tjy1XRDRWTHQzVGZDbVk/edit?usp=sharing

BUG=
R=dsinclair@chromium.org, nduca@chromium.org

Review URL: https://codereview.appspot.com/12751046

git-svn-id: 3a56fcae908c7e16d23cb53443ea4795ac387cf2@895 0e6d7f2b-9903-5b78-7403-59d27f066143

bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer,bpsinc-native/src_third_party_trace-viewer