Various Feedback fixes.

This CL fixes two issues.
. It brings up a scroll bar if the text in the feedback window doesn't fit in
  the space available on screen (this happens when the user sets the page zoom
  too 120% or above)
. Sets the focus to the description field when the feedback app starts.

R=xiyuan@chromium.org
BUG=295999,330139

Review URL: https://codereview.chromium.org/511393002

Cr-Commit-Position: refs/heads/master@{#292530}

chrome/browser/resources/feedback/css/feedback.css

@@ -11,6 +11,7 @@ body {
   background-color: #fbfbfb;
   height: 100%;
   margin: 0;
+  overflow: auto;
   padding: 0;
   width: 100%;
 }

chrome/browser/resources/feedback/js/feedback.js

@@ -307,6 +307,8 @@ function initialize() {
           $('histograms-url').onclick =
               windowOpener(STATS_WINDOW_ID, 'chrome://histograms');
         }
+        // Make sure our focus starts on the description field.
+        $('description-text').focus();
       });
     }
   });