Use correct viewport for intrinsic size-less SVG images in <svg:image>

Prior to https://codereview.chromium.org/403393002, an SVG image which had
no intrinsic size specified, would (somewhat accidentally) get the
<image>'s bounds set as their container size. After said CL, the container
size would never be set, and hence remain as 300x150. This will result in
incorrect scale factors being used when painting, and hence an incorrectly
scaled result.
Revert to setting the <image>'s bounds when the SVG image does not have an
intrinsic size.

BUG=411906

Review URL: https://codereview.chromium.org/551043006

git-svn-id: svn://svn.chromium.org/blink/trunk@181752 bbb929c8-8fbe-4397-9dbb-9b2b20218538