Simplify the _setScale call when changing Scroll modes

Since the current page will be explicitly scrolled into view *directly* afterwards anyway (compare with e.g. the `pagesRotation` code), trying to maintain the current position when re-applying the zoom level during changing of Scroll mode is pointless.

web/pdf_viewer.js

@@ -111,7 +111,7 @@ class PDFViewer extends BaseViewer {
     // Call this before re-scrolling to the current page, to ensure that any
     // changes in scale don't move the current page.
     if (isNaN(this._currentScaleValue)) {
-      this._setScale(this._currentScaleValue, this.isInPresentationMode);
+      this._setScale(this._currentScaleValue, true);
     }
     this.scrollPageIntoView({ pageNumber, });
     this.update();