cc: Fix layer raster tile iterator not to skip a tile erroneously.

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}

• cc/layers/picture_layer_impl.cc[diff]
• cc/layers/picture_layer_impl.h[diff]
• cc/layers/picture_layer_impl_unittest.cc[diff]