Use consistent tile coordinate keys

Tile coordinate keys from ol.tilecoord.toString() are different than the ones
from Array.prototype.toString(). Both were used, potentially causing problems.
This change makes them consistent and removes the now unnecessary
ol.tileCoord.toString() function.

src/ol/renderer/canvas/canvastilelayerrenderer.js

@@ -14,7 +14,6 @@ goog.require('ol.extent');
 goog.require('ol.layer.Tile');
 goog.require('ol.renderer.canvas.Layer');
 goog.require('ol.size');
-goog.require('ol.tilecoord');
 goog.require('ol.vec.Mat4');
 
 
@@ -336,7 +335,7 @@ ol.renderer.canvas.TileLayer.prototype.prepareFrame =
       }
       goog.asserts.assert(tile);
       if (drawableTile(tile)) {
-        tilesToDrawByZ[z][ol.tilecoord.toString(tile.tileCoord)] = tile;
+        tilesToDrawByZ[z][tile.tileCoord.toString()] = tile;
         continue;
       }
       fullyLoaded = tileGrid.forEachTileCoordParentTileRange(