Fix compiler error for Unity 5.4 or below.

Rect.zero is introduced from Unity 5.5.  Using it will cause compiler
error when compiling for Unity 5.4 or below.
Change the checking to check Rect.width and Rect.height instead.

Change-Id: I53d05e9c65bad2ae3000b8b284214b2f13dbc6ba

Author: chkuang-g

source/VersionHandlerImpl/src/DialogWindow.cs

@@ -320,7 +320,7 @@ void OnGUI() {
         // Adjust the dialog window size according to the rendered content.
         // Rect returned by BeginVertical() can be zeroes for a couple of frames, therefore
         // ignoring resizing for those frames.
-        if (rect != Rect.zero) {
+        if (rect.width != 0.0f && rect.height != 0.0f) {
             // Additional space at the bottom of the window.
             const float FILLER_WINDOWS_HEIGHT = 15.0f;
             float windowHeight = rect.height + FILLER_WINDOWS_HEIGHT;