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
We probably want something similar to the chrome check above where we look to see if we're less then a given version.