net/dns/ documentation

Update comments on HostResolverImpl, ProcTask, and DnsTask.

Change-Id: I0f9a82b1cbb87e972835ae57d475eca08f30f76d
Reviewed-on: https://chromium-review.googlesource.com/975632Reviewed-by: Bence Béky <bnc@chromium.org>
Commit-Queue: Miriam Gershenson <mgersh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545139}

net/dns/host_resolver_impl.cc

@@ -670,6 +670,9 @@ class HostResolverImpl::RequestImpl : public HostResolver::Request {
 
 // Calls HostResolverProc in TaskScheduler. Performs retries if necessary.
 //
+// In non-test code, the HostResolverProc is always SystemHostResolverProc,
+// which calls a platform API that implements host resolution.
+//
 // Whenever we try to resolve the host, we post a delayed task to check if host
 // resolution (OnLookupComplete) is completed or not. If the original attempt
 // hasn't completed, then we start another attempt for host resolution. We take
@@ -966,7 +969,11 @@ class HostResolverImpl::ProcTask
 
 //-----------------------------------------------------------------------------
 
-// Resolves the hostname using DnsTransaction.
+// Resolves the hostname using DnsTransaction, which is a full implementation of
+// a DNS stub resolver. One DnsTransaction is created for each resolution
+// needed, which for AF_UNSPEC resolutions includes both A and AAAA. The
+// transactions are scheduled separately and started separately.
+//
 // TODO(szym): This could be moved to separate source file as well.
 class HostResolverImpl::DnsTask : public base::SupportsWeakPtr<DnsTask> {
  public:

net/dns/host_resolver_impl.h

@@ -31,11 +31,11 @@ class NetLog;
 class NetLogWithSource;
 
 // For each hostname that is requested, HostResolver creates a
-// HostResolverImpl::Job. When this job gets dispatched it creates a ProcTask
-// which runs the given HostResolverProc in TaskScheduler. If requests for that
-// same host are made during the job's lifetime, they are attached to the
-// existing job rather than creating a new one. This avoids doing parallel
-// resolves for the same host.
+// HostResolverImpl::Job. When this job gets dispatched it creates a task
+// (ProcTask for the system resolver or DnsTask for the async resolver) which
+// resolves the hostname. If requests for that same host are made during the
+// job's lifetime, they are attached to the existing job rather than creating a
+// new one. This avoids doing parallel resolves for the same host.
 //
 // The way these classes fit together is illustrated by:
 //