Fix GpuLayerCacheTest compile error

Fix GpuLayerCacheTest compile error:
GpuLayerCacheTest.cpp:120:34: error: comparison between signed and
unsigned integer expressions [-Werror=sign-compare]

gcc --version
gcc (Ubuntu/Linaro 4.6.4-1ubuntu1~12.04) 4.6.4

Review URL: https://codereview.chromium.org/794003003
diff --git a/tests/GpuLayerCacheTest.cpp b/tests/GpuLayerCacheTest.cpp
index cba331f..a149c36 100644
--- a/tests/GpuLayerCacheTest.cpp
+++ b/tests/GpuLayerCacheTest.cpp
@@ -94,7 +94,7 @@
 // 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;