Adds a baseTime property to top level events to help re-normalize
timestamps for multi process compiles.
Adds some more instrumentation to profile the Compiler task.

Review at http://gwt-code-reviews.appspot.com/797801


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8643 8db76d5a-ed1c-0410-87a9-c151d255dfc7