Block NPAPI plugins by default

Add --enable-npapi flag in chrome://flags to support re-enable.

BUG=295137
TEST=browser_tests, unit_tests, content_browsertests, blink layout tests

Review URL: https://codereview.chromium.org/645203002

Cr-Commit-Position: refs/heads/master@{#312374}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index 09b672c..15baf08 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -500,6 +500,15 @@
     kOsAll,
     SINGLE_VALUE_TYPE(switches::kDisableExperimentalWebGL)
   },
+#if defined(OS_WIN) || defined(OS_MACOSX)
+  {
+    "enable-npapi",
+    IDS_FLAGS_ENABLE_NPAPI_NAME,
+    IDS_FLAGS_ENABLE_NPAPI_DESCRIPTION,
+    kOsWin | kOsMac,
+    SINGLE_VALUE_TYPE(switches::kEnableNpapi)
+  },
+#endif
   {
     "disable-webrtc",
     IDS_FLAGS_DISABLE_WEBRTC_NAME,