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On Android (and soon iOS), we track what kind of tile we show on the NTP
so that we can compare different kinds of clicks in UMA. The results are
probably no surprise: click rate for colored tiles is about twice that
of default-colored tiles (though it's not necessarily clear how much to
attribute that to our presentation and how much to the fact that good
sites generally have favicons).

Right now we have the default background color hard-coded in a few
places and use that to determine when a tile is default-colored. This is
possibly incorrect (when #787878 is in fact the dominant color) and
generally bad (constants can get out of sync).

This CL adds `bool FallbackIconStyle::is_default_background_color`, so
we can check that instead of comparing colors, and threads it through to
Android. It also deletes a few copies of the color constant. Three
copies remain: the authoritative copy in fallback_icon_style.cc, one in
its test, and one in Android's colors.xml.

BUG=651056

Review-Url: https://codereview.chromium.org/2374753002
Cr-Commit-Position: refs/heads/master@{#422739}