Add pop_back() to GrAllocator and add unit test.

gyp/tests.gypi

 # Common gypi for unit tests.
 {
   'include_dirs': [
     '../src/core',
     '../src/effects',
     '../src/image',
     '../src/lazy',
     '../src/images',
     '../src/pathops',
     '../src/pdf',
@@ skipping 90 matching lines @@
     '../tests/GLProgramsTest.cpp',
     '../tests/GeometryTest.cpp',
     '../tests/GifTest.cpp',
     '../tests/GpuColorFilterTest.cpp',
     '../tests/GpuDrawPathTest.cpp',
     '../tests/GpuLayerCacheTest.cpp',
     '../tests/GpuRectanizerTest.cpp',
     '../tests/GrBinHashKeyTest.cpp',
     '../tests/GrContextFactoryTest.cpp',
     '../tests/GrDrawTargetTest.cpp',
+    '../tests/GrAllocatorTest.cpp',
     '../tests/GrMemoryPoolTest.cpp',
     '../tests/GrOrderedSetTest.cpp',
     '../tests/GrGLSLPrettyPrintTest.cpp',
     '../tests/GrRedBlackTreeTest.cpp',
     '../tests/GrSurfaceTest.cpp',
     '../tests/GrTBSearchTest.cpp',
     '../tests/GradientTest.cpp',
     '../tests/ImageCacheTest.cpp',
     '../tests/ImageDecodingTest.cpp',
     '../tests/ImageFilterTest.cpp',
@@ skipping 97 matching lines @@
     '../src/utils/debugger/SkObjectParser.h',
     '../src/utils/debugger/SkObjectParser.cpp',
 
     '../tests/PipeTest.cpp',
     '../src/pipe/utils/SamplePipeControllers.cpp',
 
     '../tests/TDStackNesterTest.cpp',
     '../experimental/PdfViewer/src/SkTDStackNester.h',
   ],
 }