Set chrome://gpu background-color

The page text is setup for a white background, which is normally the
case, but on Fuchsia the background defaults to black and chrome://gpu
is unreadable.

Bug: none
Change-Id: I803ff40a3e9e3a54fc606332537a0685c92388a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453435
Commit-Queue: kylechar <kylechar@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#814443}

content/browser/resources/gpu/gpu_internals.html

@@ -14,6 +14,7 @@ found in the LICENSE file.
 }
 
 body {
+  background-color: white;
   cursor: default;
   font-family: sans-serif;
   padding: 0;