Fix JPEGDecode downsampled output size

Do not assume the desired decode size is supported by libjpeg.
Instead query jpeg_calc_output_dimensions and use the returned
decoded size instead.

BUG=345125

Review URL: https://codereview.chromium.org/173233003

git-svn-id: svn://svn.chromium.org/blink/trunk@167620 bbb929c8-8fbe-4397-9dbb-9b2b20218538