Use different shading for off-cpu self time in the flame graph

[mstange]

Each box in the flame graph currently has two pieces:

• The "non-self" piece, where there are callee boxes stack on top.
• The "self" piece, representing a call node's self samples.

The "self" piece is always on the right.

Once we know which samples are on-cpu samples and which samples are off-cpu samples, we can split the "self" piece into two sub-pieces:

• The "on-CPU self" piece
• The "off-CPU self" piece

And then we can shade the off-CPU self piece differently, for example give it less opacity.

┆Issue is synchronized with this Jira Task

[mstange]

This would have been useful to have when we were looking at https://share.firefox.dev/41eDpVo and trying to figure out why so much time is spent in the self-time of ZwAlertThreadByThreadIdEx. In the beginning we were assuming that all of those self samples were on-CPU samples but that's probably not the case; shading would have made it more immediately obvious.