Send events when user taps download icon

To track "download" button interactions we have to notify
EventLogging that button was clicked. The easiest approach is
to use mw.track() and then in WikimediaEvents subscribe to the
`minerva.downloadAsPdf` events and track page impressions.

Bug: T181297
Change-Id: Iecbebe37c165dda3f26af47906662f6e5a81321d

resources/skins.minerva.scripts/DownloadIcon.js

@@ -1,4 +1,4 @@
-( function ( M ) {
+( function ( M, track ) {
 
 	var msg = mw.msg,
 		MAX_PRINT_TIMEOUT = 3000,
@@ -47,13 +47,18 @@
 
 			function doPrint() {
 				self.timeout = clearTimeout( self.timeout );
+				track( 'minerva.downloadAsPDF', {
+					action: 'callPrint'
+				} );
 				window.print();
 				hideSpinner();
 			}
-
 			// The click handler may be invoked multiple times so if a pending print is occurring
 			// do nothing.
 			if ( !this.timeout ) {
+				track( 'minerva.downloadAsPDF', {
+					action: 'fetchImages'
+				} );
 				this.showSpinner();
 				// If all image downloads are taking longer to load then the MAX_PRINT_TIMEOUT
 				// abort the spinner and print regardless.
@@ -71,4 +76,4 @@
 	} );
 
 	M.define( 'skins.minerva.scripts/DownloadIcon', DownloadIcon );
-}( mw.mobileFrontend ) );
+}( mw.mobileFrontend, mw.track ) );

resources/skins.minerva.scripts/init.js

@@ -1,4 +1,4 @@
-( function ( M, $ ) {
+( function ( M, track, $ ) {
 	var
 		config = mw.config,
 		toast = M.require( 'mobile.startup/toast' ),
@@ -245,8 +245,11 @@
 			// left-most in the display. Since we want the download button to be to
 			// the left of the watchstar, we put it after it in the DOM.
 			new DownloadIcon( skin ).$el.insertAfter( '#ca-watch' );
+			track( 'minerva.downloadAsPDF', {
+				action: 'buttonVisible'
+			} );
 		}
 	} );
 
 	M.define( 'skins.minerva.scripts/overlayManager', overlayManager );
-}( mw.mobileFrontend, jQuery ) );
+}( mw.mobileFrontend, mw.track, jQuery ) );