[iOS] Add smooth scrolling fullscreen viewport adjustment choice.

Bug: none
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I088c5e5cbd24998539aef88eea1a034cb3604891
Reviewed-on: https://chromium-review.googlesource.com/1173714Reviewed-by: Peter Lee <pkl@chromium.org>
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582803}

ios/chrome/browser/ui/fullscreen/fullscreen_features.h

@@ -17,7 +17,7 @@ extern const char kViewportAdjustmentExperimentCommandLineSwitch[];
 // The available viewport adjustment experiments.  The choices in this array
 // correspond with the ViewportAdjustmentExperiment values.
 extern const flags_ui::FeatureEntry::Choice
-    kViewportAdjustmentExperimentChoices[4];
+    kViewportAdjustmentExperimentChoices[5];
 
 // Enum type describing viewport adjustment experiments.
 enum class ViewportAdjustmentExperiment : short {
@@ -28,6 +28,8 @@ enum class ViewportAdjustmentExperiment : short {
               // container view.
   HYBRID,  // Translates the web view up and down and updates the viewport using
            // safe area insets.
+  SMOOTH_SCROLLING,  // Adjusts the viewport using the smooth scrolling
+                     // workaround.
 };
 
 // Convenience method for retrieving the active viewport adjustment experiment

ios/chrome/browser/ui/fullscreen/fullscreen_features.mm

@@ -16,6 +16,7 @@ namespace {
 const char kContentInsetChoiceValue[] = "content-inset";
 const char kSafeAreaChoiceValue[] = "safe-area";
 const char kHybridChoiceValue[] = "hybrid";
+const char kSmoothScrollingChoiceValue[] = "smooth";
 }
 
 namespace fullscreen {
@@ -32,7 +33,8 @@ const flags_ui::FeatureEntry::Choice kViewportAdjustmentExperimentChoices[] = {
      "safe-area"},
     {"Use Hybrid Implementation",
      kViewportAdjustmentExperimentCommandLineSwitch, "hybrid"},
-};
+    {"Use Smooth Scrolling Workaround",
+     kViewportAdjustmentExperimentCommandLineSwitch, "smooth"}};
 
 ViewportAdjustmentExperiment GetActiveViewportExperiment() {
   const base::CommandLine* command_line =
@@ -46,6 +48,8 @@ ViewportAdjustmentExperiment GetActiveViewportExperiment() {
       return ViewportAdjustmentExperiment::SAFE_AREA;
     if (viewport_experiment == std::string(kHybridChoiceValue))
       return ViewportAdjustmentExperiment::HYBRID;
+    if (viewport_experiment == std::string(kSmoothScrollingChoiceValue))
+      return ViewportAdjustmentExperiment::SMOOTH_SCROLLING;
   }
   return ViewportAdjustmentExperiment::FRAME;
 }