Make default font size in Files.app slightly larger.

This CL changes default font size to comply with the design spec.

BUG=402344
TEST=none

Review URL: https://codereview.chromium.org/902663002

Cr-Commit-Position: refs/heads/master@{#314992}

ui/file_manager/file_manager/foreground/css/file_manager.css

@@ -48,6 +48,7 @@ body {
   display: flex;
   flex: auto;
   flex-direction: column;
+  font-size: 81%;
   height: 100%;
   margin: 0;
   padding: 0;