Add GrResourceAllocator class + unit test

Change-Id: I2700e8cb4213479b680519ba67f078cc3fb71376
Reviewed-on: https://skia-review.googlesource.com/23661
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
diff --git a/gn/tests.gni b/gn/tests.gni
index fde7df6..37ccfca 100644
--- a/gn/tests.gni
+++ b/gn/tests.gni
@@ -187,6 +187,7 @@
   "$_tests/RefDictTest.cpp",
   "$_tests/RegionTest.cpp",
   "$_tests/RenderTargetContextTest.cpp",
+  "$_tests/ResourceAllocatorTest.cpp",
   "$_tests/ResourceCacheTest.cpp",
   "$_tests/RoundRectTest.cpp",
   "$_tests/RRectInPathTest.cpp",