Disable line ending modifications by git in .png files

Change-Id: Ib569f8587f18f91b89b4b8b824fa99cf1a5e6005
Reviewed-on: https://chromium-review.googlesource.com/1115359Reviewed-by: Sergiy Byelozyorov <sergiyb@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54062}

.gitattributes

 # Automatically normalize line endings (to LF) for all text-based files.
 * text=auto eol=lf
+# Do not modify line endings for binary files (which are sometimes auto
+# detected as text files by git).
+*.png binary