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Rather than plumbing around an int64_t for color, using a magic value to
represent an invalid/null color, just use base::Optional.

This simplifies the code in a number of ways:
- No more magic values
- No need to assume that all non-magic int64_t values fit in a uint32_t
- No more reinterpret_cast<uint32_t&>

The conversion helpers are consolidated into color_helpers.h and shared
as well.

Bug: 836683
Change-Id: I09ac717f3c852c63680a2c1c561ee8e2fed98777
Reviewed-on: https://chromium-review.googlesource.com/1050994
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Mounir Lamouri <mlamouri@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559366}