Linux: Call memset() in a couple places in ExceptionHandler to avoid uninit memory reads under Valgrind.

Also move private static variables into the .cc file.

BUG=chromium:332335
R=ivanpe@chromium.org

Review URL: https://breakpad.appspot.com/5734002

git-svn-id: http://google-breakpad.googlecode.com/svn/trunk@1385 4c0a9323-5329-0410-9bdc-e9ce6186880e