[Code Health] Reformat throughput_test_case.py

The format in this file is in-consistent, sometimes there is one
blank line after a test case, and sometimes it is two. I looked at
other test cases and it seems two blank lines is the right format.
This CL does the re-format, no beavior change.

TBR=sadrul@chromium.org

Bug: None
Change-Id: I6e6246eb9278ed5bd5acd3d9bdd86d0468cc78f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041798Reviewed-by: Xida Chen <xidachen@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738983}

tools/perf/page_sets/rendering/throughput_test_cases.py

@@ -90,30 +90,35 @@ class MainSixtyImplZero(ThroughputMetricStory):
   URL = ('file://../../../../chrome/test/data/perf/throughput_test_cases/'
          'main-animations-throughput.html#60')
 
+
 class MainFifteenWithJankImplZero(ThroughputMetricStory):
   BASE_NAME = 'main_15fps_with_jank_impl_0fps'
   SUPPORTED_PLATFORMS = platforms.ALL_PLATFORMS
   URL = ('file://../../../../chrome/test/data/perf/throughput_test_cases/'
          'main-animations-throughput.html?jank#15')
 
+
 class MainSixtyWithJankImplZero(ThroughputMetricStory):
   BASE_NAME = 'main_60fps_with_jank_impl_0fps'
   SUPPORTED_PLATFORMS = platforms.ALL_PLATFORMS
   URL = ('file://../../../../chrome/test/data/perf/throughput_test_cases/'
          'main-animations-throughput.html?jank#60')
 
+
 class MainZeroWithJankImplZero(ThroughputMetricStory):
   BASE_NAME = 'main_0fps_with_jank_impl_0fps'
   SUPPORTED_PLATFORMS = platforms.ALL_PLATFORMS
   URL = ('file://../../../../chrome/test/data/perf/throughput_test_cases/'
          'main-animations-throughput.html?jank#0')
 
+
 class OffScreenMainSixty(ThroughputMetricStory):
   BASE_NAME = 'off_screen_main_60fps'
   SUPPORTED_PLATFORMS = platforms.ALL_PLATFORMS
   URL = ('file://../../../../chrome/test/data/perf/throughput_test_cases/'
          'main-animations-throughput.html?offscreen#60')
 
+
 class OffScreenMainSixtyJank(ThroughputMetricStory):
   BASE_NAME = 'off_screen_main_60fps_jank'
   SUPPORTED_PLATFORMS = platforms.ALL_PLATFORMS