[Android] Conditionally enable M text selection features

Enable longpress drag selection and smart selection by default on
Android platforms more recent than L, for both Chrome and WebView.

BUG=466749,451255

Review URL: https://codereview.chromium.org/1175703002

Cr-Commit-Position: refs/heads/master@{#333781}

content/browser/android/content_startup_flags.cc

@@ -63,6 +63,14 @@ void SetContentCommandLineFlags(bool single_process,
   parsed_command_line->AppendSwitch(switches::kEnableOverlayScrollbar);
   parsed_command_line->AppendSwitch(switches::kValidateInputEventStream);
 
+  // TODO(jdduke): Use the proper SDK version when available, crbug.com/466749.
+  if (base::android::BuildInfo::GetInstance()->sdk_int() >
+      base::android::SDK_VERSION_LOLLIPOP_MR1) {
+    parsed_command_line->AppendSwitch(switches::kEnableLongpressDragSelection);
+    parsed_command_line->AppendSwitchASCII(
+        switches::kTouchTextSelectionStrategy, "direction");
+  }
+
   // There is no software fallback on Android, so don't limit GPU crashes.
   parsed_command_line->AppendSwitch(switches::kDisableGpuProcessCrashLimit);