MemoryCache::resourceForURL(): add argument validation.

Source/core/fetch/MemoryCacheTest.cpp

Index: Source/core/fetch/MemoryCacheTest.cpp
diff --git a/Source/core/fetch/MemoryCacheTest.cpp b/Source/core/fetch/MemoryCacheTest.cpp
index e40765292665db5ecb73f40f4fd84ca2dc318951..57e567d0bd02adbd7828f0f688808322f9e3c0b3 100644
--- a/Source/core/fetch/MemoryCacheTest.cpp
+++ b/Source/core/fetch/MemoryCacheTest.cpp
@@ -554,6 +554,7 @@ TEST_F(MemoryCacheTest, ResourceMapIsolation)
     EXPECT_EQ(resource1.get(), memoryCache()->resourceForURL(url));
     EXPECT_EQ(resource1.get(), memoryCache()->resourceForURL(url, memoryCache()->defaultCacheIdentifier()));
     EXPECT_EQ(resource2.get(), memoryCache()->resourceForURL(url, "foo"));
+    EXPECT_EQ(0, memoryCache()->resourceForURL(KURL()));
 
     ResourcePtr<FakeResource> resource3 = new FakeResource(ResourceRequest("http://test/resource"), Resource::Raw);
     resource3->setCacheIdentifier("foo");