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We're currently always drawing tiled backgrounds in repeat/repeat mode,
even if we need tiling in one dimension only.

This has the unexpected side effect of opposite edge bleed, in the
dimension which doesn't require tiling, when rasterized for hidpi
devices (DSF causing Skia to sample across the tile boundary).

Detect cases where tile repetition is not needed, and strength-reduce the
shader tile mode accordingly.

Also make Image::drawPattern() protected, as it is only used in subclasses.

BUG=673261
R=reed@google.com,fs@opera.com,schenney@chromium.org

Review-Url: https://codereview.chromium.org/2582383002
Cr-Commit-Position: refs/heads/master@{#439529}