Fix comments related to InsertionPoint::attach

I originally thought we could avoid attaching out of order, but it turns
out that we need to do that because the n^2 protection inside
NodeRenderingContext will make insertion points add their renderers in
the wrong place if we don't.

This patch fixes the comments.

Review URL: https://codereview.chromium.org/68243005

git-svn-id: svn://svn.chromium.org/blink/trunk@161753 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unused DefaultFilterOperation and GammaFilterOperation.

DefaultFilterOperation was added for GraphicsLayerCA. It has never used for
Chromium. [1]

Gamma filter was removed in [2]. GammaFilterOperation class is a leftover.

[1] http://src.chromium.org/viewvc/blink?view=revision&revision=107422
[2] http://src.chromium.org/viewvc/blink?view=revision&revision=103125

BUG=none
TEST=none; no behavior changes

Review URL: https://codereview.chromium.org/68773003

git-svn-id: svn://svn.chromium.org/blink/trunk@161754 bbb929c8-8fbe-4397-9dbb-9b2b20218538