Bug 1014113 - Refine and enable zoom for find in page. r=margaret

Defines a viewport factor around the highlighted word
May need more testing (See bug 1015395)

--HG--
extra : rebase_source : 81f45a18f8ca8c9a3787522805933a1f801efc00

mobile/android/chrome/content/FindHelper.js

@@ -150,8 +150,21 @@ var FindHelper = {
         this._targetTab.sendViewportUpdate();
       }
     } else {
-      // Disabled until bug 1014113 is fixed
-      // ZoomHelper.zoomToRect(aData.rect);
+      // Defines the space around the highlighted element as a factor of the element's size.
+      const spacingFactor = 6;
+
+      // We replace the start of the zoom rect to keep the highlighted word in the middle.
+      // We divide this offset by two to consider a spacing on each side of the rect.
+      let x = aData.rect.x + (aData.rect.width * (1 - spacingFactor)) / 2;
+      let y = aData.rect.y + (aData.rect.height * (1 - spacingFactor)) / 2;
+
+      let rect = new Rect(Math.max(x, 0),
+                          Math.max(y, 0),
+                          // we use a bigger viewport than just the highlighted word
+                          aData.rect.width * spacingFactor,
+                          aData.rect.height * spacingFactor);
+
+      ZoomHelper.zoomToRect(rect);
       this._viewportChanged = true;
     }
   }

mobile/android/chrome/content/ZoomHelper.js

@@ -123,6 +123,11 @@ var ZoomHelper = {
    * if it is really tall.
    */
   zoomToRect: function(aRect, aClickY = -1) {
+    if(aRect.isEmpty()) {
+      // Protect from empty or negative-sized rects & potentials NaN in following calculations
+      return;
+    }
+
     let viewport = BrowserApp.selectedTab.getViewport();
 
     let rect = {