Fix ftrace event explorer
Fix ftrace event explorer

Fix a couple of issues with explorer.html
- There are now so many ftrace format files that we hit some limit in
  Chrome trying to load them all simultaneously so manually limit
  the numbers of outstanding requests to a small number.
- Remove withAttr since it is no longer supporter by mithril.
- Rename JsDiff -> Diff to match the package change.

Test: ./src/traced/probes/ftrace/test/explorer.html
Change-Id: I5ffcc827ea25a3964251c45e550a8358289364ad
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto