Correct task items' layout for RTL languages.

This CL fixes task items' layout problem in action button (as combobutton) and default task (in context menu).
In the screenshot attached in issue 371232, these problems are pointed as "Icon position of task menu is strange.". 
For margin and padding, I used -webkit-margin-start etc..
For background position, I used separate selector for html in 'rtl'.  

BUG=371232

Review URL: https://codereview.chromium.org/278793002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@269546 0039d316-1c4b-4281-b951-d872f2087c98

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

@@ -8,16 +8,20 @@
 }
 
 .buttonbar .combobutton > .action {
+  -webkit-padding-start: 21px;
   background-position: left center;
   background-repeat: no-repeat;
   background-size: 16px 16px;
-  padding-left: 21px;
+}
+
+html[dir='rtl'] .buttonbar .combobutton > .action {
+  background-position: right center;
 }
 
 .buttonbar .combobutton > .trigger {
-  border-left: solid 1px #dcdcdc;
-  margin-left: 8px;
-  margin-right: -8px;
+  -webkit-border-start: solid 1px #dcdcdc;
+  -webkit-margin-end: -8px;
+  -webkit-margin-start: 8px;
   width: 22px;
 }
 

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

@@ -194,6 +194,10 @@ menu.chrome-menu > :not(hr) {
   padding-right: 20px;
 }
 
+html[dir='rtl'] menu.chrome-menu > :not(hr) {
+  background-position: left 10px center;
+}
+
 menu.chrome-menu > .menuitem-button {
   background-position: center;
   background-repeat: no-repeat;