Update HTTPHeaderMap wrappers to use AtomicString type for header values

Update HTTPHeaderMap wrappers to use AtomicString type for header values instead
of String, for consistency with HTTPHeaderMap. This avoids uselessly constructing
Strings and then having to convert it to an AtomicString, which has a cost.

A lot of callers already have an AtomicString or a string literal, so we end up
constructing AtomicStrings from Strings a lot less.

HTTPHeaderMap uses AtomicString type for both header names and values to save
memory, as there are a lot of identical strings.

R=abarth, eseidel
BUG=323739

Review URL: https://codereview.chromium.org/99733002

git-svn-id: svn://svn.chromium.org/blink/trunk@163027 bbb929c8-8fbe-4397-9dbb-9b2b20218538