Add image/webp in Blink's default accept header

This is to cover scenarios such as:
- Inline images (part of the HTML)
- Direct request for an image URL (Blink request to this URL uses
  default accept header, as it isn't sure about the type of the
  resource at request time).

BUG=267212

Review URL: https://chromiumcodereview.appspot.com/21687003

git-svn-id: svn://svn.chromium.org/blink/trunk@155457 bbb929c8-8fbe-4397-9dbb-9b2b20218538