Modify LayoutTests to prep for compositing solid background scollers

third_party/WebKit/LayoutTests/compositing/layer-creation/fixed-position-in-fixed-overflow.html

Index: third_party/WebKit/LayoutTests/compositing/layer-creation/fixed-position-in-fixed-overflow.html
diff --git a/third_party/WebKit/LayoutTests/compositing/layer-creation/fixed-position-in-fixed-overflow.html b/third_party/WebKit/LayoutTests/compositing/layer-creation/fixed-position-in-fixed-overflow.html
index 95e85dc8c19afbe27a8105346bdeb8158812cedb..f08ccd41260879b0f730ac14060c0cf557f9e35b 100644
--- a/third_party/WebKit/LayoutTests/compositing/layer-creation/fixed-position-in-fixed-overflow.html
+++ b/third_party/WebKit/LayoutTests/compositing/layer-creation/fixed-position-in-fixed-overflow.html
@@ -19,6 +19,7 @@
   width: 100px;
   height: 1000px;
   background-color: blue;

[flackr, 2016/08/03 20:00:01]

Should background color still be here? The image will completely cover it up
right?

[Stephen Chennney, 2016/08/03 20:21:15]

Yes, it will cover it up, but we only use background color in determining
composited scrolling, so I want to leave it in.

A gradient with opaque colors completely fills a space if there is no
background-image-size given or other things to reduce coverage, but we don't
check for that right now. 

[flackr, 2016/08/03 20:34:14]

Perhaps I am missing something. .blue is not a scroller, and we won't yet check
whether the entire scrolling contents has opaque backgrounds afaik. Also, why do
we want this to become a composited scroller if it's not today - i.e. keeping it
not composited would preserve the current test result. I thought the gradient
was just to be able to visually tell where you were in the scroll.

[Stephen Chennney, 2016/08/03 20:44:26]

.blue is a scroller if the window size is less than 1000px, as it is in layout
tests. My interest was in manual testing the correctness of things when it is
composited, but in hindsight that's inconsistent unless I also change the
.overflow height to be something explicitly less than 1000px.

And I want it to be composited so that we're testing the user's experience (or
will be).

I would go with fixing the size so that the test works manually in any window
size, but I'd also be satisfied just entirely reverting this change. Do you have
any preference?

+  background-image: linear-gradient(to bottom,red,orange,yellow, green, blue,indigo,violet);
 }
 </style>
 <div class="overflow fixed">