I am so sorry that you are going through this with your little one. It's so sad to see them get old and develop health issues when they do absolutely nothing but make our lives happy. It just doesn't seem fair. :(
The tremors that you mention shouldn't be coming from the phenobarbital. The usual side effects in canines from phenobarbital use are lethargy, polydipsia (increased thirst), polyuria (increased urination), polyphagia (increased appetite), anxiety and weight gain. From the symptoms I think your vet is probably right on the money with the diagnosis of a mass in the brain, but I doubt that the tremors signify pain because you said they are of short duration. Pain wouldn't be that transient, so it sounds like she may be having a neurological reaction to the mass.
As long as she is eating her food, eager to go outside and is alert and happy to see you when you talk to her, just let her be. You will know when it's time. She will tell you, in her own way, that she is ready and the signs will be unmistakable. In the meantime, feed her whatever she wants to eat and spoil her beyond all reason. Maybe take her for rides to places that she enjoys going and just sit and take in the scenery together.
Please keep us up to date on her condition.
Hi. It is also just possible and worth discounting that the tremors are minor seizures, possibly due to an inappropriately lower than needed dose of phenobarbital. While the likelihood is as Ghilly has mentioned (that a brain tumor may be causing neurological symptoms), it is certainly worth raising this with your vet. Instead of raising the phenobarbital, it may be worth adding potassium bromide, which acts to improve the actions of the primary anticonvulsant drug.
I hope that Tony is correct, that would be great if you could control the tremors by either increasing her dosage or adding potassium bromide. Please stop back and let us know what your vet says.