Add a comment to the ThirdPartyModulesBlocking feature flag

The comment explains that 2 browser starts are required to fully enable
or disable the feature.

Bug: 846953
Change-Id: I44037ab9d1c8b3a1c3f2692916ad945fedfa2b2e
Reviewed-on: https://chromium-review.googlesource.com/1142345
Commit-Queue: Chris Hamilton <chrisha@chromium.org>
Reviewed-by: Chris Hamilton <chrisha@chromium.org>
Cr-Commit-Position: refs/heads/master@{#576214}

chrome/common/chrome_features.cc

@@ -149,6 +149,8 @@ const base::Feature kTabMetricsLogging{"TabMetricsLogging",
 
 #if defined(OS_WIN) && defined(GOOGLE_CHROME_BUILD)
 // Enables the blocking of third-party modules.
+// Note: Due to a limitation in the implementation of this feature, it is
+// required to start the browser two times to fully enable or disable it.
 const base::Feature kThirdPartyModulesBlocking{
     "ThirdPartyModulesBlocking", base::FEATURE_DISABLED_BY_DEFAULT};
 #endif