support content_shell

chromium content_shell for linux uses different version string.
the string is fixed and not changed automatically in content_shell.gypi.
('content_shell_version': '19.77.34.5')
trace-viewer checks the chrome version from UA whether the browser
is higher than 36. This CL makes trace-viewer check content_shell_version too.

BUG=674
Actually, this won't work correctly. You need to know if this is content shell and do the right check. But you only want to do one of the checks in each case. Checking <36 and <19 doesn't make sense. We should work with any version of content shell that is later then 19.77 but not before (assuming that's the version where we started with webcomponents).