Android: Animation effect for 'Open In a New Tab' button for Preview Tab

This CL gives animation (alpha) effect to the 'Open in a New Tab' button
on preview tab header when sliding up/down the panel.

Bug: 1047889
Change-Id: I28504f8fd89d8cfe67d5523bdd5e2f186000cbd1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2032458
Commit-Queue: Jinsuk Kim <jinsukkim@chromium.org>
Reviewed-by: Donn Denman <donnd@chromium.org>
Cr-Commit-Position: refs/heads/master@{#737687}

chrome/android/java/src/org/chromium/chrome/browser/compositor/bottombar/ephemeraltab/EphemeralTabCoordinator.java

@@ -81,7 +81,6 @@ public class EphemeralTabCoordinator implements View.OnLayoutChangeListener {
             @Override
             public void onSheetStateChanged(int newState) {
                 if (mSheetContent == null) return;
-                mSheetContent.showOpenInNewTabButton(newState == SheetState.FULL);
                 switch (newState) {
                     case SheetState.PEEK:
                         mMetrics.recordMetricsForPeeked();
@@ -101,9 +100,9 @@ public class EphemeralTabCoordinator implements View.OnLayoutChangeListener {
 
             @Override
             public void onSheetOffsetChanged(float heightFraction, float offsetPx) {
-                if (heightFraction == 0.0f) {
-                    mMetrics.recordMetricsForClosed(mCloseReason);
-                }
+                if (mSheetContent == null) return;
+                if (heightFraction == 0.0f) mMetrics.recordMetricsForClosed(mCloseReason);
+                mSheetContent.showOpenInNewTabButton(heightFraction);
             }
         });
     }

chrome/android/java/src/org/chromium/chrome/browser/compositor/bottombar/ephemeraltab/EphemeralTabSheetContent.java

@@ -191,10 +191,19 @@ public class EphemeralTabSheetContent implements BottomSheetContent {
         progressBar.setVisibility(visible ? View.VISIBLE : View.GONE);
     }
 
-    /** Called to show or hide the open in new tab button. */
-    public void showOpenInNewTabButton(boolean show) {
-        View openInNewTabButton = mToolbarView.findViewById(R.id.open_in_new_tab);
-        openInNewTabButton.setVisibility(show ? View.VISIBLE : View.GONE);
+    /**
+     * Called to show (with alpha) or hide the open in new tab button.
+     * @param fraction Alpha for the button when visible.
+     */
+    public void showOpenInNewTabButton(float fraction) {
+        View button = mToolbarView.findViewById(R.id.open_in_new_tab);
+        // Start showing the button about halfway toward the full state.
+        if (fraction <= 0.5f) {
+            if (button.getVisibility() != View.GONE) button.setVisibility(View.GONE);
+        } else {
+            if (button.getVisibility() != View.VISIBLE) button.setVisibility(View.VISIBLE);
+            button.setAlpha((fraction - 0.5f) * 2.0f);
+        }
     }
 
     @Override