Fix length calculation in imagesizes when <meta> viewport is present

This fixes a bug where "vw" length calculation of imagesizes attribute
coming from Link rel=preload header did not reflect the content
width from <meta name=viewport>.

MediaValuesDynamic::ComputeLength() did not use overridden viewport
dimensions.

Bug: 813452
Change-Id: Icbc8f124e552b681bfffa80f407543078f851ed3
Reviewed-on: https://chromium-review.googlesource.com/c/1365051
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614594}
3 files changed