Fix broken Pepper Lock Mouse.

Testing Pepper Lock Mouse (for appropriate cleanup of RenderWidget)
revealed I accidentally removed a line in
https://chromium-review.googlesource.com/c/chromium/src/+/2446593.

Verified with out/Release/chrome
--register-pepper-plugins="out/Release/libppapi_example_mouse_lock.so;application/x-ppapi-example-mouse-lock" ppapi/examples/mouse_lock/mouse_lock.html

BUG=1134674

Change-Id: Ibce92521985f6dbde4f8a210169d8f6fe5198912
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453903Reviewed-by: Bill Budge <bbudge@chromium.org>
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814412}

content/renderer/pepper/pepper_plugin_instance_impl.cc

@@ -2664,7 +2664,9 @@ int32_t PepperPluginInstanceImpl::LockMouse(
   if (!HasTransientUserActivation())
     return PP_ERROR_NO_USER_GESTURE;
 
-  // Either mouselock succeeded or a Flash fullscreen is pending.
+  if (!LockMouse(false))
+    return PP_ERROR_FAILED;
+
   lock_mouse_callback_ = callback;
   return PP_OK_COMPLETIONPENDING;
 }