Fixed warning with "DOMAgent.RGBA"
by fixing externs "protocol_externs.js" in "compile_frontend.py" and
JSDoc descriptions

BUG=233496
R=aandrey@chromium.org, pfeldman@chromium.org

Review URL: https://codereview.chromium.org/23874009

git-svn-id: svn://svn.chromium.org/blink/trunk@157766 bbb929c8-8fbe-4397-9dbb-9b2b20218538