Polymer elements added to third_party/polymer.

Polymer elements added to third_party/polymer.

List of changes:
* Added all core-elements and paper-elements provided by Polymer.
* Components moved to 'components' subdirectory.
* Added a presubmit check verifying that the list of dependencies from
  bower.json and the list of components installed to 'components' are the same.
  (see README.chromium for details).
* Updated README.chromium
* Updated pathes to polymer.js and platform.js in projects that use them.

web-animations-js library will be added to the repository
later. Here is a list of components depending on web-animations-js:
* core-animation/core-animation-group.html
* core-animation/core-animation.html
* core-animation/web-animations.html
* paper-menu-button/paper-menu-button.html
* paper-menu-button/paper-menu-button-transition.html

This CL doesn't change polymer and platform components.

Landed manually, because of the bug in the commit queue (http://crbug.com/416255).

BUG=415696
R=mtomasz@chromium.org, raymes@chromium.org, rsadam@chromium.org, thestig@chromium.org
TBR=cpu

Review URL: https://codereview.chromium.org/582873003

Cr-Commit-Position: refs/heads/master@{#296712}