Add feature and flag for enabling DOH in DNS proxy.

When this feature is enabled, SecureDNS provider configuration will be
processed and propogated to Shill via the SecureDnsManager.

BUG=b:182240110
TEST=unit_tests --gtest_filter='AboutFlagsHistogramTest.*'

Change-Id: I036a5f0debd87cd4171861af7b58183b1b422f1f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2854242
Reviewed-by: Steven Bennetts <[email protected]>
Reviewed-by: Kyle Horimoto <[email protected]>
Commit-Queue: Garrick Evans <[email protected]>
Cr-Commit-Position: refs/heads/master@{#880839}
diff --git a/chrome/browser/flag_descriptions.cc b/chrome/browser/flag_descriptions.cc
index 6515792..cc4551e 100644
--- a/chrome/browser/flag_descriptions.cc
+++ b/chrome/browser/flag_descriptions.cc
@@ -4229,6 +4229,12 @@
     "When enabled, standard DNS queries will be proxied through the system "
     "service";
 
+const char kDnsProxyEnableDOHName[] =
+    "Enable DNS-over-HTTPS in the DNS proxy service";
+const char kDnsProxyEnableDOHDescription[] =
+    "When enabled, the DNS proxy will perform DNS-over-HTTPS in accordance "
+    "with the Chrome OS SecureDNS settings.";
+
 const char kEnableEncryptionMigrationName[] =
     "Enable encryption migration of user data";
 const char kEnableEncryptionMigrationDescription[] =