pull mipmap class into its own (private) header

BUG=
R=scroggo@google.com

Review URL: https://codereview.chromium.org/19462007

git-svn-id: http://skia.googlecode.com/svn/trunk@10161 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gyp/tests.gyp b/gyp/tests.gyp
index c6dd1c6..e93a12f 100644
--- a/gyp/tests.gyp
+++ b/gyp/tests.gyp
@@ -77,6 +77,7 @@
         '../tests/Matrix44Test.cpp',
         '../tests/MemsetTest.cpp',
         '../tests/MetaDataTest.cpp',
+        '../tests/MipMapTest.cpp',
         '../tests/OSPathTest.cpp',
         '../tests/PackBitsTest.cpp',
         '../tests/PaintTest.cpp',