DevTools: Network html preview should always have a white background
DevTools: Network html preview should always have a white background

The iframe doesnt change colors due to dark theme, so the background
needs to stay white as well. Otherwise there is dark text on a
dark background.

Bug: 763919
Change-Id: I8a542dc613044bd8ac3d18dfccec66b65010ba0e
Reviewed-on: https://chromium-review.googlesource.com/c/1271943
Reviewed-by: Dmitry Gozman <0ca14b32cbb359a30ac562af27dd84cc2813eacb@chromium.org>
Commit-Queue: Joel Einbinder <bab755abe9fee48fe544272e55f73517d4e4a725@chromium.org>
Cr-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#598399}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: c67340779f137602c4cb91ef5bfbaebc63226eed
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