Use modern base APIs to avoid base::MessageLoop::current() calls in /net

This is a precusor cleanup extracted from
https://chromium-review.googlesource.com/c/chromium/src/+/957760/7
in the broader effort to restrict usage of MessageLoop::current().

This CL was uploaded by git cl split.

R=jkarlin@chromium.org

Bug: 825327
Change-Id: I5cf12a4e5b2c9867b6b4006b8b42ca0363d5481c
Reviewed-on: https://chromium-review.googlesource.com/986814Reviewed-by: Josh Karlin <jkarlin@chromium.org>
Commit-Queue: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#547721}

net/dns/dns_transaction_unittest.cc

@@ -789,8 +789,7 @@ class DnsTransactionTest : public testing::Test {
 
   void TearDown() override {
     // Check that all socket data was at least written to.
-    if (base::MessageLoop::current() &&
-        base::MessageLoop::current()->IsType(base::MessageLoop::TYPE_IO)) {
+    if (base::MessageLoopForIO::IsCurrent()) {
       URLRequestFilter* filter = URLRequestFilter::GetInstance();
       filter->ClearHandlers();
     }