Fix SpeedTracer Log when compilation is spread across different processes.

The common time referrence recorded in "baseTime" was not correct.

Change-Id: If91008cd75c187dff667adc2411f60d4dadabbe3
Review-Link: https://gwt-review.googlesource.com/#/c/2031/

Review by: mdempsky@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11533 8db76d5a-ed1c-0410-87a9-c151d255dfc7