Writing suppression.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/918639b05b7d426d803dc4536993880f9bfc475a

commit 918639b05b7d426d803dc4536993880f9bfc475a
Author: rjkroege <rjkroege@chromium.org>
Date: Thu Apr 14 22:34:21 2016

Mark flaky .../animation-timeline/document-timeline.html

imported/web-platform-tests/web-animations/animation-timeline/document-timeline.html
is failing intermittently on Linux

BUG= 603703 
TBR=suzyh@chromium.org

Review URL: https://codereview.chromium.org/1887913003

Cr-Commit-Position: refs/heads/master@{#387453}

[modify] https://crrev.com/918639b05b7d426d803dc4536993880f9bfc475a/third_party/WebKit/LayoutTests/TestExpectations

Thanks, I'll look into it with the others in  crbug.com/600248