Use AtomicString type more consistently for mimeType / encoding

Use AtomicString type more consistently for mimeType / encoding to avoid unnecessary conversions
from String to AtomicString, as those have a cost.

AtomicString type was already used for mimeType / encoding in ResourceResponse and this CL
updates ArchiveResource to be consistent, as well as the related code.

This is one step towards making the AtomicString(const String&) constructor explicit.

R=abarth, eseidel
BUG=323739

Review URL: https://codereview.chromium.org/99333006

git-svn-id: svn://svn.chromium.org/blink/trunk@163186 bbb929c8-8fbe-4397-9dbb-9b2b20218538