Replace viewport height with percent

This allows other elements to occupy screen real estate.
The html, body is already set to height 100%, so the presentation
stays the same.

Change-Id: I019088da80b4632b48f7089518950f620a0e00f3

polygerrit-ui/app/elements/gr-app.html

@@ -43,7 +43,7 @@ limitations under the License.
     <style>
       :host {
         display: flex;
-        min-height: 100vh;
+        min-height: 100%;
         flex-direction: column;
       }
       gr-main-header,