Split Twitter placeholder flags. (#17527)

- Use a separate flag for each aspect of the default placeholder so that
we can try different combinations.

extensions/amp-twitter/0.1/amp-twitter.css

@@ -55,7 +55,7 @@ amp-twitter {
   height: 100%;
 }
 
-.i-amphtml-twitter-pulse .i-amphtml-twitter-placeholder-svg  {
+.i-amphtml-twitter-pulse:not([i-amphtml-loading]) .i-amphtml-twitter-placeholder-svg  {
   transition-property: transform, opacity;
   transition-duration: 100ms;
   transition-timing-function: ease-in;
@@ -97,7 +97,7 @@ amp-twitter[i-amphtml-loading="done"] .i-amphtml-twitter-placeholder-svg {
   opacity: 0;
 }
 
-.i-amphtml-twitter-pulse[i-amphtml-loading="done"] .i-amphtml-twitter-placeholder-svg  {
+.i-amphtml-twitter-burst[i-amphtml-loading="done"] .i-amphtml-twitter-placeholder-svg  {
   transition-duration: 250ms;
   transition-timing-function: ease-out;
   transform: scale(12);

extensions/amp-twitter/0.1/amp-twitter.js

@@ -31,7 +31,7 @@ import {setStyle} from '../../../src/style';
  *    plusing.
  */
 function isPulsingPlaceholder(win) {
-  return isExperimentOn(win, 'twitter-default-placeholder');
+  return isExperimentOn(win, 'twitter-default-placeholder-pulse');
 }
 
 /**
@@ -45,19 +45,19 @@ function isFadingPlaceholder(win) {
 
 /**
  * @param {!Window} win
- * @return {boolean} True if we should show the default placeholder.
+ * @return {boolean} True if we should show the default placeholder with
+ *    a burst ending animation.
  */
-function isStaticPlaceholder(win) {
-  return isExperimentOn(win, 'twitter-default-placeholder-static');
+function isBurstingPlaceholder(win) {
+  return isExperimentOn(win, 'twitter-default-placeholder-burst');
 }
 
 /**
  * @param {!Window} win
  * @return {boolean} True if we should show the default placeholder.
  */
 function enableDefaultPlaceholder(win) {
-  return isPulsingPlaceholder(win) || isStaticPlaceholder(win) ||
-      isFadingPlaceholder(win);
+  return isExperimentOn(win, 'twitter-default-placeholder');
 }
 
 class AmpTwitter extends AMP.BaseElement {
@@ -135,6 +135,9 @@ class AmpTwitter extends AMP.BaseElement {
     if (isFadingPlaceholder(this.win)) {
       this.element.classList.add('i-amphtml-twitter-fade');
     }
+    if (isBurstingPlaceholder(this.win)) {
+      this.element.classList.add('i-amphtml-twitter-burst');
+    }
     this.updateForLoadingState_();
     listenFor(iframe, MessageType.EMBED_SIZE, data => {
       this.updateForSuccessState_(data['height']);