Add comment to note we should change FileReader::m_loadingState before firing events

A follow-up patch to https://chromiumcodereview.appspot.com/23745005/

BUG=237800
TEST=none

Review URL: https://chromiumcodereview.appspot.com/23545021

git-svn-id: svn://svn.chromium.org/blink/trunk@157126 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Make CSSHostRule.addRule() / deleteRule() arguments mandatory

Make CSSHostRule.addRule() / deleteRule() arguments mandatory as per the
specification:
https://dvcs.w3.org/hg/webcomponents/raw-file/tip/spec/shadow/index.html#css-host-rule-interface

It is confusing otherwise what is supposed to happen when the arguments
are omitted. Firefox / IE do not implement CSSHostRule yet.

Review URL: https://chromiumcodereview.appspot.com/23531015

git-svn-id: svn://svn.chromium.org/blink/trunk@157127 bbb929c8-8fbe-4397-9dbb-9b2b20218538
DevTools: Inspector should be informed when viewport rule creation failed.

The test added here is correct but passes even without this change because tests are run with experimental web platform features enabled by default.

BUG=278289

Review URL: https://chromiumcodereview.appspot.com/23514032

git-svn-id: svn://svn.chromium.org/blink/trunk@157128 bbb929c8-8fbe-4397-9dbb-9b2b20218538