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Fix the constructor of the layer raster tile iterator to never advance
a tiling raster tile iterator. This fixes the iterator not to skip the
first tile returned by a tiling raster tile iterator even if there are
no NOW tiles but only SOON and/or EVENTUALLY tiles.

Review URL: https://codereview.chromium.org/555183002

Cr-Commit-Position: refs/heads/master@{#294188}