Add a pref name for share targets, and store their manifest data.

Adds a dictionary pref name, to store sites with manifests that
contain the share_url_template field; the key is the site origin, and
value is the value of share_url_template in the site's manifest.

Reads manifests after they have been parsed, for the share_url_template,
and stores the site and value under the pref name added.

Follow-up CL will read the pref store for the pref name added, and read
share targets in chrome/browser/webshare/share_service_impl.cc.

NO_DEPENDENCY_CHECKS=true
BUG=668389

Review-Url: https://codereview.chromium.org/2639463002
Cr-Commit-Position: refs/heads/master@{#447721}
diff --git a/chrome/browser/BUILD.gn b/chrome/browser/BUILD.gn
index efb12a13..f91e7cb 100644
--- a/chrome/browser/BUILD.gn
+++ b/chrome/browser/BUILD.gn
@@ -1324,6 +1324,8 @@
     "web_data_service_factory.h",
     "webshare/share_service_impl.cc",
     "webshare/share_service_impl.h",
+    "webshare/share_target_pref_helper.cc",
+    "webshare/share_target_pref_helper.h",
     "win/app_icon.cc",
     "win/app_icon.h",
     "win/browser_util.cc",