IANAL, but it seems that one could make a passable argument for tortious interference[1]. Google isn't just affecting their B2B relationship with Symantec, they're using their share in the browser market to affect Symantec's relationship with Symantec's customers.
I'm supportive of Google in this fight, but I really don't think Google would have an argument to counter-sue. Tortious interference isn't just having an effect on the relationship, you also need to have an actual tort involved. In this case Symantec could argue that Google was exaggerating the negative PR, and Symantec would probably have an easier time proving damages (from customers leaving due to their certificates being phased out). I'm not sure what tort Google could claim in response that Symantec performed. Maybe issuing the unauthorized test certificates for Google's domains? (some sort of fraud?) But IM(NAL)O that's a tough sell.
I'd argue that by advertising browser compatibility[1], meeting the browser trust requirements is implicitly part of the business relationship, thus Google enforcing them does not give rise to tortious interference. (FWIW, I studied English law, but that was some years ago and I am largely unfamiliar with tortious interference)
[1] https://en.m.wikipedia.org/wiki/Tortious_interference