fix(mdInput): fix regression that broke input placeholders on Firefox

Change to only hide automatic placeholders on webkit.
This fixes Firefox so that placeholders are visible again.

Fixes #2162.  Fixes #2480.

Author: Splaktar

src/components/input/input.scss

@@ -264,4 +264,4 @@ md-input-container.md-icon-float {
   md-input-container.md-default-theme > md-icon {
     fill: #fff;
   }
-}
+}

src/components/input/input.spec.js

@@ -186,4 +186,4 @@ describe('md-input-container directive', function() {
     var input = el.find('input');
     expect(input.attr('aria-label')).toBe('baz');
   }));
-});
+});