Allow smaller minimum-scale for image documents

The default minimum-scale 0.25 is too big for huge image documents on
small screens. Especially for a huge image that has been down-sampled
during decoding to meet memory constraint, at the current minimum-scale
the image may look blurred and too big.

Explicitly specify minimum-scale=0.1 for image documents.

This affects only platforms (e.g. Android) on which meta-viewport is
supported and shrinksStandaloneImagesToFit is disabled.

BUG=338404
TEST=fast/images/viewport-in-standalone-image-document.html

Review URL: https://codereview.chromium.org/148493002

git-svn-id: svn://svn.chromium.org/blink/trunk@165985 bbb929c8-8fbe-4397-9dbb-9b2b20218538