Implement AMP-side click measurement. (ampproject#2969)

Introduces a redirect-avoidance mechanism that enables referrers to track a link invocation without an intermediate redirect by passing a URL to AMP via a fragment param. This change is experiment guarded.

Also introduces a small new binary for use on the referrer side (e.g. inside of an ad creative) that helps with constructing the respectives URLs and preconnects to AMP.

Primary implementation of ampproject#2934

add custom closure compiler runner (ampproject#2935)

Expose the initial intersection info as a variable. (ampproject#2907)

This makes simple use cases that only want to read this value simpler to implement (and avoids immediate unlistening).

Add on to ampproject#2878

Revert "Remove ad during unlayout" (ampproject#2898)

This reverts commit 03850ee.

We suspect that this negatively affects UX by unloading during accidental swipes. Rollforward will improve heuristic.

tools(changelog): handle non merge commits

Merge pull request ampproject#2772 from jessedijkstra/patch-1

Change the spelling of "availbale" to "available"

Merge pull request ampproject#2688 from powdercloud/amp-accordion-doc…

…umentation-update

Update the amp-accordion documentation

Ensure that all extensions are processed

Merge pull request ampproject#2625 from erwinmombay/fix-cyc-changelog

fix changelog to handle PR's that have a merge coming from master

Merge pull request ampproject#2566 from dvoytenko/access56

Ensure that pingback is executed when directly requested