Remove the setting of default accept header for main resources

This CL removes the setting of the default accept header in
FrameFetchContext::addAdditionalRequestHeaders for main resources.
https://chromiumcodereview.appspot.com/313313002/ sets it in
RenderFrameImpl::willSendRequest.

BUG=376025

Review URL: https://codereview.chromium.org/315073005

git-svn-id: svn://svn.chromium.org/blink/trunk@175655 bbb929c8-8fbe-4397-9dbb-9b2b20218538