Use base::StartWith() in more places when appropriate.

media/base/key_systems.cc

Index: media/base/key_systems.cc
diff --git a/media/base/key_systems.cc b/media/base/key_systems.cc
index e970f0a474a79ca3512b54432d44a8057baa3453..4567bf4c614a12bb8d46c1f758f44e302c5434ca 100644
--- a/media/base/key_systems.cc
+++ b/media/base/key_systems.cc
@@ -169,10 +169,8 @@ static bool IsPotentiallySupportedKeySystem(const std::string& key_system) {
   // can use the "x-" prefix to avoid conflicting with and advertising support
   // for real key system names. Use is discouraged.
   const char kExcludedPrefix[] = "x-";
-  if (key_system.find(kExcludedPrefix, 0, arraysize(kExcludedPrefix) - 1) == 0)
-    return true;
-
-  return false;
+  return base::StartsWith(key_system, kExcludedPrefix,
+                          base::CompareCase::SENSITIVE);
 }
 
 class KeySystemsImpl : public KeySystems {