Remove external SkImageFilter cache, and rename UniqueIDCache -> Cache.

There Can Only Be One.... Cache for SkImageFilter.

R=bsalomon@google.com

BUG=skia:

Author: senorblanco@chromium.org

Review URL: https://codereview.chromium.org/452923002
diff --git a/src/core/SkBitmapDevice.cpp b/src/core/SkBitmapDevice.cpp
index 8c92f71..5f205ce 100644
--- a/src/core/SkBitmapDevice.cpp
+++ b/src/core/SkBitmapDevice.cpp
@@ -367,8 +367,8 @@
     return NULL;
 }
 
-SkImageFilter::UniqueIDCache* SkBitmapDevice::getImageFilterCache() {
-    SkImageFilter::UniqueIDCache* cache = SkImageFilter::UniqueIDCache::Get();
+SkImageFilter::Cache* SkBitmapDevice::getImageFilterCache() {
+    SkImageFilter::Cache* cache = SkImageFilter::Cache::Get();
     cache->ref();
     return cache;
 }