simpleperf: fix report_html.py.

report_html.py removes threads having too small percentage
of samples. But it doesn't remove processes having no threads,
which makes showing flamegraph broken.

Bug: none
Test: run test.py TestReportHtml.test_no_empty_process.
Change-Id: I3d568c98f0426a8ebbc9760dbc3c2bd25eda4c67
3 files changed