Invert the flag for prefixed EME.

Once the Blink change lands, the prefixed EME APIs will be disabled by default.

They can be enabled with the enable-prefixed-encrypted-media flag.

BUG=451117

Review URL: https://codereview.chromium.org/1327113002

Cr-Commit-Position: refs/heads/master@{#348198}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index b3756b3..b834407 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -742,11 +742,11 @@
      IDS_FLAGS_DISABLE_ENCRYPTED_MEDIA_DESCRIPTION,
      kOsAll,
      SINGLE_VALUE_TYPE(switches::kDisableEncryptedMedia)},
-    {"disable-prefixed-encrypted-media",
-     IDS_FLAGS_DISABLE_PREFIXED_ENCRYPTED_MEDIA_NAME,
-     IDS_FLAGS_DISABLE_PREFIXED_ENCRYPTED_MEDIA_DESCRIPTION,
+    {"enable-prefixed-encrypted-media",
+     IDS_FLAGS_ENABLE_PREFIXED_ENCRYPTED_MEDIA_NAME,
+     IDS_FLAGS_ENABLE_PREFIXED_ENCRYPTED_MEDIA_DESCRIPTION,
      kOsAll,
-     SINGLE_VALUE_TYPE(switches::kDisablePrefixedEncryptedMedia)},
+     SINGLE_VALUE_TYPE(switches::kEnablePrefixedEncryptedMedia)},
     {"disable-javascript-harmony-shipping",
      IDS_FLAGS_DISABLE_JAVASCRIPT_HARMONY_SHIPPING_NAME,
      IDS_FLAGS_DISABLE_JAVASCRIPT_HARMONY_SHIPPING_DESCRIPTION,