Support 64 bit argument types

[ghost]

For full compatibility with C++, it would be great if we could support 64 bit argument types. I imagine that this will be easy to do in the wire format but may be tricky to preserve the full range for display. It is probably a pre-requisite for supporting nanosecond-based 64bit timestamps too.

[benvanik]

Though supporting these in the UI will be difficult, supporting them in the file format should just require changes here:
https://github.com/google/tracing-framework/blob/master/src/wtf/data/variable.js#L80
and here:
https://github.com/google/tracing-framework/blob/master/src/wtf/db/eventtypebuilder.js#L254