Add Blink support for showing image placeholders using range requests.

Design doc:
https://docs.google.com/document/d/1691W7yFDI1FJv69N2MEtaSzpnqO2EqkgGD3T0O-pQ08/edit?usp=sharing

This CL introduces support for issuing a range request for just the
first few bytes of an image, and showing a translucent gray box of the
same size as the image in the image's place if the original image's
dimensions can be decoded from the returned range.

Currently, this behavior is only activated via a blink-settings flag. In
the future, features such as Data Saver will activate this functionality
to save data for users.

BUG=605350,605351

Review-Url: https://codereview.chromium.org/2423683002
Cr-Commit-Position: refs/heads/master@{#427141}
18 files changed