Expose fontScale to JS

Libraries/Utilities/Dimensions.js

@@ -29,6 +29,7 @@ if (dimensions && dimensions.windowPhysicalPixels) {
     width: windowPhysicalPixels.width / windowPhysicalPixels.scale,
     height: windowPhysicalPixels.height / windowPhysicalPixels.scale,
     scale: windowPhysicalPixels.scale,
+    fontScale: windowPhysicalPixels.fontScale,
   };
 
   // delete so no callers rely on this existing

Libraries/Utilities/PixelRatio.js

@@ -59,6 +59,20 @@ class PixelRatio {
     return Dimensions.get('window').scale;
   }
 
+  /**
+   * Returns the scaling factor for font sizes. This is the ratio that is used to calculate the
+   * absolute font size, so any elements that heavily depend on that should use this to do
+   * calculations.
+   *
+   * If a font scale is not set, this returns the device pixel ratio.
+   *
+   * Currently this is only implemented on Android and reflects the user preference set in
+   * Settings > Display > Font size, on iOS it will always return the default pixel ratio.
+   */
+  static getFontScale(): number {
+    return Dimensions.get('window').fontScale || PixelRatio.get();
+  }
+
   /**
    * Converts a layout size (dp) to pixel size (px).
    *