simpleperf: Remove small stack gaps in gecko_profile_generator.py

Samples having broken call stack split views in Stack Chart page in
firefox profiler. To improve this, we remove samples contributing to
small stack gaps in Stack Chart.

By default, we remove stack gaps with length <= 3. But this can be
controlled by --remove-gaps. For example, we can stop removing gaps
by using `--remove-gaps 0`.

Bug: 270628163
Test: run test.py
Change-Id: I8f4af55828b35122a2828e78f70ad9571016ac02
2 files changed