simpleperf: fix the height of generated flamegraphs.

In FlameGraphView, maxDepth can be much bigger than needed. Because
_renderSvgNodesWithSameRoot() cuts nodes with width < 0.1%. So fix
the function calculating maxDepth.

Bug: none
Test: run report_html.py manually.
Change-Id: I21e76b15d123db80d2cac3fa9bbc7337d8beb7a6
1 file changed