Run testing/buildbot/generate_buildbot_json.py

I get a presubmit error when trying to upload patches otherwise.
Seems to be an error from r678539.

Bug: 932205
Change-Id: I95a657fa4269990df5a4079657209eaf2bcb31b5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1707349
Auto-Submit: Matt Falkenhagen <falken@chromium.org>
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Vlad Tsyrklevich <vtsyrklevich@chromium.org>
Reviewed-by: Takuto Ikuta <tikuta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#678593}

testing/buildbot/chromium.linux.json

@@ -5993,7 +5993,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ]
         },

testing/buildbot/tryserver.chromium.linux.json

@@ -477,7 +477,7 @@
           "can_use_on_swarming_builders": true,
           "dimension_sets": [
             {
-              "os": "Ubuntu-14.04"
+              "os": "Ubuntu-16.04"
             }
           ]
         },