perf-turbo.py merges a perf data file and a turbofan trace file into a
single json object which can then be piped to a file and uploaded to
turbolizer to display the profiling data in the disassembly. With the
changes, turbolizer now shows the event counts for instruction in
percentage form and with heatmap-stype colouring. Multiple different
events can be recorded at once with a new drop-down menu to select which
event to view the counts of. The documentation has been updated with
instructions. Using the script is optional and turbolizer retains
previous functionality if a trace without profiling data is uploaded.

BUG=None

Review-Url: https://codereview.chromium.org/2174803002
Cr-Commit-Position: refs/heads/master@{#38124}