Flip the true/false result of ScriptState::contextIsValid

Currently ScriptState::contextIsValid returns true when the context is not valid.
This is very confusing. This CL flips the true/false.

This CL doesn't fix bug 419624.
This is just a preparation for fixing bug 419624 (actually I have no idea at the moment though).

BUG=419624
TBR=jochen

Review URL: https://codereview.chromium.org/622333002

git-svn-id: svn://svn.chromium.org/blink/trunk@183271 bbb929c8-8fbe-4397-9dbb-9b2b20218538