Fix some accessibility issues in image editing.

ui/file_manager/gallery/css/gallery.css

Index: ui/file_manager/gallery/css/gallery.css
diff --git a/ui/file_manager/gallery/css/gallery.css b/ui/file_manager/gallery/css/gallery.css
index 1863a16c99fae4b3daa5384cf2166b2856ed7a65..3e977c0436dc2056852194c1b313ec695580422c 100644
--- a/ui/file_manager/gallery/css/gallery.css
+++ b/ui/file_manager/gallery/css/gallery.css
@@ -821,19 +821,20 @@ button[disabled] {
 
 .edit-modal-wrapper .container {
   justify-content: center;
+  left: 0;
+  position: absolute;
+  top: 0;
 }
 
 .edit-modal-wrapper .action-buttons {
   justify-content: flex-end;
-  left: 0;
-  position: absolute;
-  top: 0;
 }
 
 .edit-modal button.edit-toolbar {
   background: transparent;
   border: 0 none;
   border-image: none;
+  border-radius: 2px;
   box-sizing: border-box;
   color: white;
   min-width: 0;
@@ -841,6 +842,10 @@ button[disabled] {
   position: relative;
 }
 
+.edit-modal button.edit-toolbar:focus {
+  background-color: rgba(255, 255, 255, 0.12);

[oka, 2016/10/06 09:11:55]

Should we use 
rgba(0, 0, 0, 0.3) for consistency?
The EXIT button in the lower right corner in the edit toolbar is highlighted
with this value if focused.

https://cs.chromium.org/chromium/src/ui/file_manager/gallery/css/gallery.css?...

[fukino, 2016/10/06 14:34:30]

The background of this button is darker, so (0, 0, 0) is almost invisible.
I picked this value from Material Design button spec for dark background.
https://material.google.com/components/buttons.html#buttons-raised-buttons

(BTW, it says "Focused:12% #000000 shade over the color", but I suppose the
correct spec is "Focused:12% #FFFFFF shade over the color".)

+}
+
 .edit-modal button.edit-toolbar .icon {
   display: none;
 }