[Benchmarking] Define OS dimension value for linux-perf-fyi

Currently no bot can be found for linux-perf-fyi builder. The reason is
that the only one linux device in the pool has the OS version as 'Ubuntu-14.04'
However, while there's no specific os dimension value defined in
perf_data_generator.py, a default value 'Ubuntu-16.04' from the infra was
used to look for a bot.

As as fix, we now specify the os value 'Ubuntu-14.04' in perf_data_generator.py
so that it will override the default and be used to look for the correct bot.

Bug: chromium:1027147
Change-Id: Ib59d881c9aceecf6a4aa6e3710bc3219f07b990c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1931144
Commit-Queue: Caleb Rouleau <crouleau@chromium.org>
Reviewed-by: Caleb Rouleau <crouleau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#718885}

testing/buildbot/chromium.perf.fyi.json

@@ -163,6 +163,7 @@
             {
               "gpu": "10de",
               "id": "build186-b7",
+              "os": "Ubuntu-14.04",
               "pool": "chrome.tests.perf-fyi"
             }
           ],

tools/perf/core/perf_data_generator.py

@@ -154,6 +154,7 @@ FYI_BUILDERS = {
     'dimension': {
       'gpu': '10de',
       'id': 'build186-b7',
+      'os': 'Ubuntu-14.04',
       'pool': 'chrome.tests.perf-fyi',
     },
   },