Implemented xdebug html tracing output

Summary: This diff implements html output for xdebug tracing. xdebug's html output
doesn't support all the features of human-readable tracing output. It would
be easy to add more features, but this diff just implements a version that
matches xdebug.

It looks like this: https://www.facebook.com/pxlcld/lrjH

Reviewed By: @ptarjan

Differential Revision: D1459347