Move image_diff code into testing/image_diff/

It is not actually a sample program to demonstrate how to use PDFium
APIs. It is more of a test utility program.

Remove header include dependency from samples/ to core/, to set a good
example.

Change-Id: Idd5d567c82314bcf0e4a7bec8a3210e7d57226ee
Reviewed-on: https://pdfium-review.googlesource.com/6871
Reviewed-by: dsinclair <dsinclair@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/testing/image_diff/DEPS b/testing/image_diff/DEPS
new file mode 100644
index 0000000..4bd2335
--- /dev/null
+++ b/testing/image_diff/DEPS
@@ -0,0 +1,5 @@
+include_rules = [
+  '+third_party/libpng16',
+  '+third_party/zlib',
+]
+