handle changed -webkit-fill-available semantics

Chromium changed how this works to better match the evolving
specification and our usage effectively became a no-op.

static/grapheneos.css

@@ -1,6 +1,10 @@
 /* use the standard Material Light theme to match GrapheneOS apps */
 /* https://android.googlesource.com/platform/frameworks/base/+/android-9.0.0_r35/core/res/res/values/colors_material.xml */
 
+html {
+    height: -webkit-fill-available;
+}
+
 body {
     display: flex;
     flex-direction: column;