Use arrayBuffer instead of text for profileURL

[f-hj]

When using #profileURL, some binary characters cannot be read if we use fetch text. So I changed that to use arrayBuffer.

Now we can read pprof protobuf files and normal JSON files instead of only text files.

[coveralls]

Coverage decreased (-0.02%) to 44.459% when pulling 1d36657 on f-hj:f-hj/fix-pprof-profileurl into 0fe0c45 on jlfwong:master.

[jlfwong]

Whoops! Nice find @f-hj! Thanks for identifying the problem and submitting a working PR fix to it 😄