Add revalidation_only option to precache config.

Add revalidation_only option to precache config.

When specified, resources are only fetched over the network if they have
an existing cache entry with revalidation headers. This serves as a
lower-bound approximation of the efficacy of a client-only wifi precache
mechanism, as it is limited to URLs that are only on the client AND in
the manifests.

BUG=679533
Review-Url: https://codereview.chromium.org/2614403003
Cr-Commit-Position: refs/heads/master@{#442999}
(cherry picked from commit 8586921e08baafc1209e66223d0642d6246094b9)

Review-Url: https://codereview.chromium.org/2623263006 .
Cr-Commit-Position: refs/branch-heads/2924@{#748}
Cr-Branched-From: 3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}
Committed: https://chromium.googlesource.com/chromium/src/+/dfc79d6a2692eede9e158d649e42a8e9567b65f0

[twifkak, 2017-01-12 22:42:56]

Description was changed from

==========
Add revalidation_only option to precache config.

When specified, resources are only fetched over the network if they have
an existing cache entry with revalidation headers. This serves as a
lower-bound approximation of the efficacy of a client-only wifi precache
mechanism, as it is limited to URLs that are only on the client AND in
the manifests.

BUG=679533

Review-Url: https://codereview.chromium.org/2614403003
Cr-Commit-Position: refs/heads/master@{#442999}
(cherry picked from commit 8586921e08baafc1209e66223d0642d6246094b9)
==========

to

==========
Add revalidation_only option to precache config.

When specified, resources are only fetched over the network if they have
an existing cache entry with revalidation headers. This serves as a
lower-bound approximation of the efficacy of a client-only wifi precache
mechanism, as it is limited to URLs that are only on the client AND in
the manifests.

BUG=679533

Review-Url: https://codereview.chromium.org/2614403003
Cr-Commit-Position: refs/heads/master@{#442999}
(cherry picked from commit 8586921e08baafc1209e66223d0642d6246094b9)

Review-Url: https://codereview.chromium.org/2623263006 .
Cr-Commit-Position: refs/branch-heads/2924@{#748}
Cr-Branched-From:
3a87aecc31cd1ffe751dd72c04e5a96a1fc8108a-refs/heads/master@{#433059}
Committed:
https://chromium.googlesource.com/chromium/src/+/dfc79d6a2692eede9e158d649e42...
==========

[twifkak, 2017-01-12 22:42:57]

Committed patchset #1 (id:1) manually as
dfc79d6a2692eede9e158d649e42a8e9567b65f0.