Comments: Assume Gerrit user

Automatic comments from Gerrit do not specify a user so we will make one
 up for Gerrit. Cannot make up an email address so no picture
 unfortunately :(.
Revisions: Check for possible NPE when precondition is not satisfied
Diff Viewer: Properly apply the fillViewport attribute so the diff fills
 the available screen real estate.

Change-Id: I89326da638e3fa7088d1b70fe4dd2a915b5e9d1b