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Since we can draw quads with a texel:pixel ratio < 1, linear
filtering can cause us to blend texels in that don't have
valid content if we don't clamp texture sampling to the middle
of boundary texels.

Clamping was already done for the anti-aliasing tile shaders.
This patch adds clamping to all tile shaders.

BUG=170722
NOTRY=True

Review URL: https://codereview.chromium.org/12086036

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@179752 0039d316-1c4b-4281-b951-d872f2087c98