[soundwave] Mark reference runs for each benchmark individually

Compute "the last day" from which we have data for each benchmark/bot
individually. This way, if a benchmark (test_suite) has not run in a
day or two, we fall back to the latest run we can find as the
"reference" value.

Also take the chance to drop a few columns which are not currently in
use.

Bug: 991923
Change-Id: Icc7a806aa5205dfa3dd49e320f66f1e1eae9f034
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1743629Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#685163}

tools/perf/cli_tools/soundwave/studies/__init__.py

@@ -24,7 +24,7 @@ def PostProcess(df):
   # We use all runs on the latest day for each quarter as reference.
   df['quarter'] = df['timestamp'].dt.to_period('Q')
   df['reference'] = df['timestamp'].dt.date == df.groupby(
-      'quarter')['timestamp'].transform('max').dt.date
+      ['quarter', 'test_suite', 'bot'])['timestamp'].transform('max').dt.date
 
   # Change units for values in ms to seconds, and percent values.
   df['units'] = df['units'].fillna('')
@@ -34,4 +34,8 @@ def PostProcess(df):
   is_percentage = df['units'].str.startswith('n%_')
   df.loc[is_percentage, 'value'] = df['value'] * 100
 
+  # Remove unused columns to save space in the output csv.
+  for col in ('point_id', 'chromium_rev', 'clank_rev', 'trace_url'):
+    del df[col]
+
   return df