Issue 3025: Add a buildflag/runtime switch to disable STP by default

STP can cause webompat issues, gating this feature behind off-by-default
runtime flag till the feature is thoroughly tested for webcompat.

The switches are available here:
https://github.com/brave/brave-core/pull/403/files

auditors: @bridiver, @bsclifton

lib/start.js

@@ -40,6 +40,9 @@ const start = (passthroughArgs, buildConfig = config.defaultBuildConfig, options
     // This only has meaning with MacOS and official build.
     braveArgs.push('--disable-brave-update')
   }
+  if (options.enable_smart_tracking_protection) {
+    braveArgs.push('--enable-smart-tracking-protection')
+  }
   if (options.single_process) {
     braveArgs.push('--single-process')
   }