Win32k lockdown: move to chrome://flags, add UMA, add Finch.

Supplying via command line is not sustainable because shortcuts are sometimes sanitized, so moving to a flag so a persistent state can be set.

Also add a Finch experiment to enable win32k lockdown, and an UMA metric to report when it's enabled (either by Finch or flags)

BUG=365160

Review URL: https://codereview.chromium.org/792873004

Cr-Commit-Position: refs/heads/master@{#307953}
diff --git a/chrome/browser/about_flags.cc b/chrome/browser/about_flags.cc
index f109d99b..6a48fb93 100644
--- a/chrome/browser/about_flags.cc
+++ b/chrome/browser/about_flags.cc
@@ -445,6 +445,14 @@
     kOsWin,
     SINGLE_VALUE_TYPE(switches::kDisableDirectWrite)
   },
+  {
+    "enable-win32k-renderer-lockdown",
+    IDS_FLAGS_ENABLE_WIN32K_RENDERER_LOCKDOWN_NAME,
+    IDS_FLAGS_ENABLE_WIN32K_RENDERER_LOCKDOWN_DESCRIPTION,
+    kOsWin,
+    ENABLE_DISABLE_VALUE_TYPE(switches::kEnableWin32kRendererLockDown,
+                              switches::kDisableWin32kRendererLockDown)
+  },
 #endif
   {
     "enable-experimental-canvas-features",