Borrow definition of Visual Viewport from CSSOM-View

Signed-off-by: Shubham Gupta <shubham.gupta@chromium.org>

Author: shubhamg13

css-viewport-1/Overview.bs

@@ -106,16 +106,25 @@ difference between the [=initial viewport=] and the [=layout viewport=].
 		This is the viewport you get after processing the viewport
 		<code>&lt;meta&gt;</code> tag.
 	</dd>
+	<dt><dfn>visual viewport</dfn></dt>
+	<dd>
+		The portion of the layout viewport that is currently visible. 
+		This is the result of applying scale transform to its layout viewport. Refer CSSOM-View <a href="https://drafts.csswg.org/cssom-view-1/#visualViewport">Visual Viewport.</a>
+		
+		Note: The [=visual viewport=] can be smaller than the [=layout viewport=]. 
+	</dd>
 </dl>
 
-ISSUE: Define visual viewport.
-
 When the [=layout viewport=] cannot fit inside the window or
 viewing area, either because the [=layout viewport=] is
 larger than the [=initial viewport=] or the zoom factor
 causes only parts of the [=layout viewport=] to be visible,
 the UA should offer a scrolling or panning mechanism.
 
+Note: The scale transform of the [=visual viewport=] is often referred to as "pinch-zoom". Conceptually, this transform
+changes the size of the CSS <a>reference pixel</a> and changes the size of the layout viewport (inversally) proportionally so that it
+does not cause reflow of the page's contents.
+
 It is recommended that initially the upper-left corners of the
 [=layout viewport=] and the window or viewing area are aligned if the
 base direction of the document is ltr. Similarly, that the upper-right