Hand cursor on hover close & back buttons

BUG=478428
TEST=Hover mouse over Back and Close buttons in new GAIA login (for Add person,
Create new account, etc).  Cursor should change to pointer hand.

Review URL: https://codereview.chromium.org/1097053008

Cr-Commit-Position: refs/heads/master@{#326857}

chrome/browser/resources/chromeos/login/oobe_screen_oauth_enrollment_webview.css

@@ -158,6 +158,10 @@ html[dir=rtl] #oauth-enroll-cancel-button {
   background-image: url(chrome://theme/IDR_LOGIN_CLOSE_BUTTTON_ON_WHITE);
 }
 
+#oauth-enroll-cancel-button:hover {
+  cursor: pointer;
+}
+
 #oauth-enroll-back-button {
   background: transparent none;
   background-image: url(chrome://theme/IDR_LOGIN_BACK_BUTTTON);
@@ -181,6 +185,10 @@ html[dir=rtl] #oauth-enroll-back-button {
   display: none;
 }
 
+#oauth-enroll-back-button:hover {
+  cursor: pointer;
+}
+
 .oauth-enroll-textbox {
   display: inline-block;
   position: relative;

chrome/browser/resources/chromeos/login/screen_gaia_signin.css

@@ -100,6 +100,10 @@ html[dir=rtl] #close-button-item {
   background-image: url(chrome://theme/IDR_LOGIN_CLOSE_BUTTTON_MASKED);
 }
 
+#close-button-item:hover {
+  cursor: pointer;
+}
+
 #back-button-item {
   background: transparent none;
   background-image: url(chrome://theme/IDR_LOGIN_BACK_BUTTTON);
@@ -127,6 +131,10 @@ html[dir=rtl] #back-button-item {
   background-image: url(chrome://theme/IDR_LOGIN_BACK_BUTTTON_MASKED);
 }
 
+#back-button-item:hover {
+  cursor: pointer;
+}
+
 .signin-text {
   color: #666;
   margin-top: 20px;