Address EventRewriter Warning
The change https://codereview.chromium.org/
653423004 introduced a warning. The Simple Chrome workflow treats all ChromeOS warnings as errors (https://sites.google.com/a/chromium.org/dev/chromium-os/how-tos-and-troubleshooting/building-chromium-browser?pli=1)
This change initializes the variable to a default unknown value.
BUG=424596
TEST=Compiled with Simple Chrome workflow. Ran existing EventRewriterTest
Review URL: https://codereview.chromium.org/
677093002
Cr-Commit-Position: refs/heads/master@{#301117}