Well, I'm somewhere in between illustrating and advocating (EDIT: “advocating” here as “advocating as legally true” rather than “advocating as desirable”); I'm generally dubious that there is a meaningful legally cognizable boundary of “own time vs. work time” as opposed to “own activity vs. work activity” for salaried employees paid for work product, as opposed to employees paid for hours of work. If you are working on something within the scope of what you are contracted to do for your employer, it would seem to be, ipso facto, work time.

EDIT: Moreover, I think that essentially what Google is doing here is having you sign away things which they otherwise could, and would were the assignment provision not permitted to them, simply prohibit outright via a during-employment non-compete, which even California (with it's unusually firm stance against post-employment non-compete) is generally fine with.