Stack sampling profiler: enable at 10% for GPU process on canary

chrome/common/stack_sampling_configuration.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_COMMON_STACK_SAMPLING_CONFIGURATION_H_
 #define CHROME_COMMON_STACK_SAMPLING_CONFIGURATION_H_
 
 #include <string>
 
 #include "base/callback.h"
@@ skipping 28 matching lines @@
   // Add a command line switch that instructs the child process to run the
   // profiler. This should only be called from the browser process.
   void AppendCommandLineSwitchForChildProcess(
       const std::string& process_type,
       base::CommandLine* command_line) const;
 
   // Returns the StackSamplingConfiguration for the process.
   static StackSamplingConfiguration* Get();
 
  private:
+  // Configuration to use for this Chrome instance.
   enum ProfileConfiguration {
     // Chrome-wide configurations set in the browser process.
     PROFILE_DISABLED,
     PROFILE_CONTROL,
     PROFILE_BROWSER_PROCESS,
     PROFILE_GPU_PROCESS,
     PROFILE_BROWSER_AND_GPU_PROCESS,
 
     // Configuration set in the child processes, which receive their enable
     // state on the command line from the browser process.
     PROFILE_FROM_COMMAND_LINE
   };
 
+  // Configuration variations, along with weights to use when randomly choosing
+  // one of a set of variations.
+  struct Variation {
+    ProfileConfiguration config;
+    int weight;
+  };
+
+  // Randomly chooses a configuration from the weighted variations. Weights must
+  // sum to 100.

[Ilya Sherman, 2016/10/12 20:52:31]

Why are weights required to sum to 100?  What if you want to run a 33%/33%/33%
trial?  (This restriction isn't present in the //base variations code, which is
why I'm asking why it's needed here.)

[Mike Wittman, 2016/10/12 21:50:41]

It's not strictly needed, but is a small sanity check. If I need a configuration
that doesn't work with this in the future I'll remove the constraint and DCHECK
at that point.

[Ilya Sherman, 2016/10/12 22:16:17]

Got it, that makes sense.  Would be nice to briefly document somewhere that this
is done as a config sanity-check =)

[Mike Wittman, 2016/10/12 23:00:32]

Done.

+  static ProfileConfiguration ChooseConfiguration(
+      const std::vector<Variation>& variations);
+
   // Generates sampling profiler configurations for all processes.
   static ProfileConfiguration GenerateConfiguration();
 
   // In the browser process this represents the configuration to use across all
   // Chrome processes. In the child processes it is always
   // PROFILE_FROM_COMMAND_LINE.
   const ProfileConfiguration configuration_;
 
   DISALLOW_COPY_AND_ASSIGN(StackSamplingConfiguration);
 };
 
 #endif  // CHROME_COMMON_STACK_SAMPLING_CONFIGURATION_H_