Handle very large images correctly

Large images may get downscaled during decoding to keep them within
the configured memory limits. When this happens while loading a WebGL
texture, bail out because otherwise the resulting texture will not have
the correct dimensions nor contents.

Tested with Nexus 5.

BUG=356181
TEST=fast/canvas/webgl/webgl-large-texture.html
R=kbr@chromium.org

Review URL: https://codereview.chromium.org/215713002

git-svn-id: svn://svn.chromium.org/blink/trunk@170339 bbb929c8-8fbe-4397-9dbb-9b2b20218538