Refactor mipmap fn from SW IDC to shared util file

This allows GPU IDC to share this logic, which it needs to handle
display resolution scaling.

BUG=
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review-Url: https://codereview.chromium.org/2037503003
Cr-Commit-Position: refs/heads/master@{#399013}

diff --git a/cc/BUILD.gn b/cc/BUILD.gn
index bb0d4da5..70c9c23 100644
--- a/cc/BUILD.gn
+++ b/cc/BUILD.gn

@@ -444,6 +444,8 @@
     "tiles/gpu_image_decode_controller.cc",
     "tiles/gpu_image_decode_controller.h",
     "tiles/image_decode_controller.h",
+    "tiles/mipmap_util.cc",
+    "tiles/mipmap_util.h",
     "tiles/picture_layer_tiling.cc",
     "tiles/picture_layer_tiling.h",
     "tiles/picture_layer_tiling_set.cc",
@@ -882,6 +884,7 @@
     "test/ordered_simple_task_runner_unittest.cc",
     "test/test_web_graphics_context_3d_unittest.cc",
     "tiles/gpu_image_decode_controller_unittest.cc",
+    "tiles/mipmap_util_unittest.cc",
     "tiles/picture_layer_tiling_set_unittest.cc",
     "tiles/picture_layer_tiling_unittest.cc",
     "tiles/software_image_decode_controller_unittest.cc",