Clarify that measure() may throw a SyntaxError

[philipwalton]

Addresses #21 .

[igrigorik]

Hmm, I don't think this fully resolves the underlying issue:

• https://w3c-test.org/user-timing/invoke_with_timing_attributes.html
• https://w3c-test.org/user-timing/measure_syntax_err.any.html

We also need to throw SyntaxError if measure's name == attribute of PerformanceTiming? Per earlier discussions, I think our pan was to enumerate and freeze the list of "blacklisted" attribute names in UT, with a callout that they're mapped to old PerformanceTiming (Nav Timing L1) interface.

With above in place, I think we could refactor both mark() and measure() to call some "check entry name" algorithm that would consult the list?

[philipwalton]

@igrigorik yeah, as I explained in #22 (comment), I was confused and thought that the use of NT names was unspecified behavior that vendors all implemented, but now I see I was wrong.

I've updated this PR to use the same text in measure as is used in mark. I wasn't sure we needed a new algorithm since this was just one line, but let me know if you think that would be cleaner.

[philipwalton]

I'm closing this as my changes have increased in scope. The new changes can be tracked in #35.