Move VIEW_HISTORY_MEMORY constant to `view_history.js`

Currently this constant is present in `viewer.js`, but it is not used there at all. Instead, it is used in `view_history.js` where we have a global for it. We might as well move the constant to `view_history.js` as that is the only place where it is used, thereby removing a global and an unused constant from `viewer.js`.

web/view_history.js

@@ -14,10 +14,12 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* globals PDFJS, VIEW_HISTORY_MEMORY, Promise */
+/* globals PDFJS, Promise */
 
 'use strict';
 
+var VIEW_HISTORY_MEMORY = 20;
+
 /**
  * View History - This is a utility for saving various view parameters for
  *                recently opened files.

web/viewer.js

@@ -31,7 +31,6 @@ var DEFAULT_URL = 'compressed.tracemonkey-pldi-09.pdf';
 var DEFAULT_SCALE_DELTA = 1.1;
 var MIN_SCALE = 0.25;
 var MAX_SCALE = 10.0;
-var VIEW_HISTORY_MEMORY = 20;
 var SCALE_SELECT_CONTAINER_PADDING = 8;
 var SCALE_SELECT_PADDING = 22;
 var PAGE_NUMBER_LOADING_INDICATOR = 'visiblePageIsLoading';