[tools/perf] Enable proxy power metric on Pixel 4a bots

This CL supplies two command-line flags that are necessary to compute
the proxy power metric (based on CPU usage and power profiles).

 * --fetch-power-profile downloads the power profile from the cloud.

 * --performance-mode=keep disables the Telemetry power management,
 making it possible to collect cpu_freq events. Note that there will
 likely be a drop in observed power rails metrics after this CL lands,
 because devices will use more efficient CPU scheduling policy.

Change-Id: I9aeecd80cda1a883e1994ba70b0a439cd83f8749
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2566995Reviewed-by: John Chen <johnchen@chromium.org>
Commit-Queue: Mikhail Khokhlov <khokhlov@google.com>
Cr-Commit-Position: refs/heads/master@{#832770}

testing/buildbot/chromium.perf.json

@@ -660,7 +660,9 @@
           "--browser=android-chrome",
           "--upload-results",
           "--test-shard-map-filename=android-pixel4a_power-perf_map.json",
-          "--experimental-tbmv3-metrics"
+          "--experimental-tbmv3-metrics",
+          "--fetch-power-profile",
+          "--performance-mode=keep"
         ],
         "isolate_name": "performance_test_suite",
         "merge": {

tools/perf/core/perf_data_generator.py

@@ -527,9 +527,12 @@ BUILDERS = {
     },
     'android-pixel4a_power-perf': {
         'tests': [{
-            'isolate': 'performance_test_suite',
+            'isolate':
+            'performance_test_suite',
             'extra_args': [
                 '--experimental-tbmv3-metrics',
+                '--fetch-power-profile',
+                '--performance-mode=keep',
             ],
         }],
         'platform':