Add a feature to enable gamepad button/axis events

This CL adds a command line switch and Feature entry for enabling
event listeners to listen for gamepad button and axis inputs.

Intent to implement thread:
https://groups.google.com/a/chromium.org/forum/#!topic/blink-dev/9cfRxXx5Cis

BUG=856290

Change-Id: I5323ff7692ffb235fc3759b1305216085ed6460c
Reviewed-on: https://chromium-review.googlesource.com/1139092
Commit-Queue: Matt Reynolds <mattreynolds@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#580361}

device/gamepad/public/cpp/gamepad_features.cc

@@ -33,11 +33,32 @@ size_t OverrideIntervalIfValid(base::StringPiece param_value,
 
 }  // namespace
 
+// Enables gamepadbuttondown, gamepadbuttonup, gamepadbuttonchange,
+// gamepadaxismove non-standard gamepad events.
+const base::Feature kEnableGamepadButtonAxisEvents{
+    "EnableGamepadButtonAxisEvents", base::FEATURE_DISABLED_BY_DEFAULT};
+
+// Overrides the gamepad polling interval.
 const base::Feature kGamepadPollingInterval{"GamepadPollingInterval",
                                             base::FEATURE_DISABLED_BY_DEFAULT};
 
 const char kGamepadPollingIntervalParamKey[] = "interval-ms";
 
+bool AreGamepadButtonAxisEventsEnabled() {
+  // Check if button and axis events are enabled by a field trial.
+  if (base::FeatureList::IsEnabled(kEnableGamepadButtonAxisEvents))
+    return true;
+
+  // Check if button and axis events are enabled by a command-line flag.
+  base::CommandLine* command_line = base::CommandLine::ForCurrentProcess();
+  if (command_line &&
+      command_line->HasSwitch(switches::kEnableGamepadButtonAxisEvents)) {
+    return true;
+  }
+
+  return false;
+}
+
 size_t GetGamepadPollingInterval() {
   size_t polling_interval = kPollingIntervalMillisecondsMax;
 

device/gamepad/public/cpp/gamepad_features.h

@@ -9,9 +9,11 @@
 
 namespace features {
 
+extern const base::Feature kEnableGamepadButtonAxisEvents;
 extern const base::Feature kGamepadPollingInterval;
 extern const char kGamepadPollingIntervalParamKey[];
 
+bool AreGamepadButtonAxisEventsEnabled();
 size_t GetGamepadPollingInterval();
 
 }  // namespace features

device/gamepad/public/cpp/gamepad_switches.cc

@@ -6,6 +6,11 @@
 
 namespace switches {
 
+// Enables gamepadbuttondown, gamepadbuttonup, gamepadbuttonchange,
+// gamepadaxismove non-standard gamepad events.
+const char kEnableGamepadButtonAxisEvents[] =
+    "enable-gamepad-button-axis-events";
+
 // Overrides the gamepad polling interval. Decreasing the interval improves
 // input latency of buttons and axes but may negatively affect performance due
 // to more CPU time spent in the input polling thread.

device/gamepad/public/cpp/gamepad_switches.h

@@ -9,6 +9,7 @@ namespace switches {
 
 // All switches in alphabetical order. The switches should be documented
 // alongside the definition of their values in the .cc file.
+extern const char kEnableGamepadButtonAxisEvents[];
 extern const char kGamepadPollingInterval[];
 
 }  // namespace switches

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -555,6 +555,10 @@
       settable_from_internals: true,
       status: "experimental",
     },
+    {
+      name: "GamepadButtonAxisEvents",
+      status: "experimental",
+    },
     {
       name: "GamepadExtensions",
       status: "experimental",

tools/metrics/histograms/enums.xml

@@ -27647,9 +27647,11 @@ from previous Chrome versions.
   <int value="-1961497025" label="tint-gl-composited-content"/>
   <int value="-1961062505" label="VrBrowsingInCustomTab:disabled"/>
   <int value="-1960567385" label="KeepPrefetchedContentSuggestions:enabled"/>
+  <int value="-1958315092" label="EnableGamepadButtonAxisEvents:disabled"/>
   <int value="-1957328398" label="MacSystemShareMenu:disabled"/>
   <int value="-1956747298" label="LayeredAPI:enabled"/>
   <int value="-1956349722" label="disable-smooth-scrolling"/>
+  <int value="-1955923385" label="EnableGamepadButtonAxisEvents:enabled"/>
   <int value="-1948540128" label="disable-webrtc-hw-encoding (deprecated)"/>
   <int value="-1946595906" label="enable-push-api-background-mode"/>
   <int value="-1946522787" label="VrCustomTabBrowsing:disabled"/>