Fix GpuLayerCacheTest compile error

tests/GpuLayerCacheTest.cpp

 /*
 * Copyright 2014 Google Inc.
 *
 * Use of this source code is governed by a BSD-style license that can be
 * found in the LICENSE file.
 */
 
 #if SK_SUPPORT_GPU
 
 #include "GrContext.h"
@@ skipping 76 matching lines @@
     cache->addUse(layer);
 
     REPORTER_ASSERT(reporter, 1 == TestingAccess::Uses(layer));
 }
 
 // This test case exercises the public API of the GrLayerCache class.
 // In particular it checks its interaction with the resource cache (w.r.t.
 // locking & unlocking textures).
 // TODO: need to add checks on VRAM usage!
 DEF_GPUTEST(GpuLayerCache, reporter, factory) {
-    static const int kInitialNumLayers = 5;
+    static const unsigned kInitialNumLayers = 5;
 
     for (int i= 0; i < GrContextFactory::kGLContextTypeCnt; ++i) {
         GrContextFactory::GLContextType glCtxType = (GrContextFactory::GLContextType) i;
 
         if (!GrContextFactory::IsRenderingGLContext(glCtxType)) {
             continue;
         }
 
         GrContext* context = factory->get(glCtxType);
 
@@ skipping 132 matching lines @@
 
         picture.reset(NULL);
         cache.processDeletedPictures();
 
         REPORTER_ASSERT(reporter, TestingAccess::NumLayers(&cache) == 0);
         // TODO: add VRAM/resource cache check here
     }
 }
 
 #endif