[benchmarking] Skip Findbrowser if possible_browser is passed in

This CL allow perf_benchmark to check for available possible_browser before it tries to find one.

This is the second step of the fix for crbug/1149275#c6

Bug: chromium:1149275
Change-Id: Ifb771363ba73d4d5cb83e58ba1e8e1f703ed182a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2550180
Commit-Queue: Wenbin Zhang <wenbinzhang@google.com>
Reviewed-by: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#829773}

tools/perf/contrib/leak_detection/leak_detection.py

@@ -24,7 +24,7 @@ def CreateCoreTimelineBasedMeasurementOptions(self):
     tbm_options.AddTimelineBasedMetric('leakDetectionMetric')
     return tbm_options
 
-  def CustomizeOptions(self, options):
+  def CustomizeOptions(self, options, possible_browser=None):
     # TODO(crbug.com/936805): Note this is a hack. Perf benchmarks should not
     # override the CustomizeOptions method.
     options.browser_options.AppendExtraBrowserArgs('--js-flags=--expose-gc')

tools/perf/core/perf_benchmark.py

@@ -78,7 +78,7 @@ def SetExtraBrowserOptions(self, options):
     """To be overridden by perf benchmarks."""
     pass
 
-  def CustomizeOptions(self, finder_options):
+  def CustomizeOptions(self, finder_options, possible_browser=None):
     # Subclass of PerfBenchmark should override  SetExtraBrowserOptions to add
     # more browser options rather than overriding CustomizeOptions.
     super(PerfBenchmark, self).CustomizeOptions(finder_options)
@@ -98,7 +98,7 @@ def CustomizeOptions(self, finder_options):
     if (browser_options.browser_type != 'reference' and
         'no-field-trials' not in browser_options.compatibility_mode):
       variations = self._GetVariationsBrowserArgs(
-          finder_options, browser_options.extra_browser_args)
+          finder_options, browser_options.extra_browser_args, possible_browser)
       browser_options.AppendExtraBrowserArgs(variations)
 
       browser_options.profile_files_to_copy.extend(
@@ -132,15 +132,20 @@ def FixupTargetOS(target_os):
       return 'chromeos'
     return target_os
 
-  def _GetVariationsBrowserArgs(self, finder_options, current_args):
+  def _GetVariationsBrowserArgs(self,
+                                finder_options,
+                                current_args,
+                                possible_browser=None):
+    if possible_browser is None:
+      possible_browser = browser_finder.FindBrowser(finder_options)
+    if not possible_browser:
+      return []
+
     chrome_root = finder_options.chrome_root
     if chrome_root is None:
       chrome_root = path_module.GetChromiumSrcDir()
 
     variations_dir = os.path.join(chrome_root, 'testing', 'variations')
-    possible_browser = browser_finder.FindBrowser(finder_options)
-    if not possible_browser:
-      return []
 
     return fieldtrial_util.GenerateArgs(
         os.path.join(variations_dir, 'fieldtrial_testing_config.json'),