Reland "Enable the clang style plugin in rtc_base/"

This is a reland of Id63f0deb7b335690157ab157c35177b7836688da.

Original change's description:
> Enable the clang style plugin in rtc_base/
> 
> Enabled the plugin and cleaned up all issues it found.
> 
> Bug: webrtc:163
> Change-Id: Id63f0deb7b335690157ab157c35177b7836688da
> Reviewed-on: https://webrtc-review.googlesource.com/14660
> Commit-Queue: Steve Anton <steveanton@webrtc.org>
> Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#20401}

Bug: webrtc:163
Change-Id: I861a5fe741215115b0e7a2be9c0786836ff5376e
Reviewed-on: https://webrtc-review.googlesource.com/15040
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Commit-Queue: Steve Anton <steveanton@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#20416}
diff --git a/rtc_base/buffer_unittest.cc b/rtc_base/buffer_unittest.cc
index 2eaf6f4..5cd1889 100644
--- a/rtc_base/buffer_unittest.cc
+++ b/rtc_base/buffer_unittest.cc
@@ -359,14 +359,14 @@
 TEST(BufferTest, TestBeginEnd) {
   const Buffer cbuf(kTestData);
   Buffer buf(kTestData);
-  auto b1 = cbuf.begin();
+  auto* b1 = cbuf.begin();
   for (auto& x : buf) {
     EXPECT_EQ(*b1, x);
     ++b1;
     ++x;
   }
   EXPECT_EQ(cbuf.end(), b1);
-  auto b2 = buf.begin();
+  auto* b2 = buf.begin();
   for (auto& y : cbuf) {
     EXPECT_EQ(*b2, y + 1);
     ++b2;