* master: (236 commits)
  trim all the columns (ampproject#3894)
  Refactoring: Turn private custom element methods into functions. (ampproject#3882)
  Lower the load priority of ad shaped iframes. (ampproject#3863)
  JsDoc fix (ampproject#3892)
  Add screenshots for Opera to AMP Validator extension. (ampproject#3866)
  Fix renaming of generated JSCompiler_prototypeAlias variable. (ampproject#3887)
  fix typo in amp-sidebar.md (ampproject#3833)
  Validator Roll-up (ampproject#3885)
  [CryptoService] Leverage browser native Crypto API to hash strings. (ampproject#3850)
  Size update (ampproject#3883)
  copy amp-ad docs to builtins (ampproject#3879)
  move doc to extension (ampproject#3878)
  [amp-experiment] Exposes isDismissed() method in AmpUserNotification (ampproject#3832)
  fix action-impl warning on dist (ampproject#3867)
  Add params for microad (ampproject#3827)
  Fixed some A4A tests. (ampproject#3859)
  Updates to colanalytics vendor config for amp-analytics. (ampproject#3849)
  Changes to implement A4A (AMP ads for AMP pages) (ampproject#3534)
  Addresses comment left over from PR#3841 (ampproject#3853)
  Expose submit event with on=submit:el.action syntax. (ampproject#3739)
  ...