Corrected logic error in reading TabProcGrowth registry value.

The TabProcGrowth registry value can be a DWORD or a string. If it is a
string, it can be a numeric string (e.g., "0"), and still be treated by
Internet Explorer as if it were an actual numeric value. This change to
BrowserFactory::GetRegistryValue() correctly handles both cases,
coercing the value to a string type for comparison.