Reland "Move webrtc/{base => rtc_base}" (https://codereview.webrtc.org/2877023002)

Reland the base->rtc_base without adding stub headers (will be
done in follow-up CL). This preserves git blame history of all files.

BUG=webrtc:7634
NOTRY=True
TBR=kwiberg@webrtc.org

Change-Id: Iea3bb6f3f67b8374c96337b63e8f5aa3e6181012
Reviewed-on: https://chromium-review.googlesource.com/554611
Reviewed-by: Henrik Kjellander <kjellander@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#18821}

webrtc/rtc_base/crc32.cc

@@ -0,0 +1,52 @@
+/*
+ *  Copyright 2012 The WebRTC Project Authors. All rights reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/base/crc32.h"
+
+#include "webrtc/base/arraysize.h"
+
+namespace rtc {
+
+// This implementation is based on the sample implementation in RFC 1952.
+
+// CRC32 polynomial, in reversed form.
+// See RFC 1952, or http://en.wikipedia.org/wiki/Cyclic_redundancy_check
+static const uint32_t kCrc32Polynomial = 0xEDB88320;
+static uint32_t kCrc32Table[256] = {0};
+
+static void EnsureCrc32TableInited() {
+  if (kCrc32Table[arraysize(kCrc32Table) - 1])
+    return;  // already inited
+  for (uint32_t i = 0; i < arraysize(kCrc32Table); ++i) {
+    uint32_t c = i;
+    for (size_t j = 0; j < 8; ++j) {
+      if (c & 1) {
+        c = kCrc32Polynomial ^ (c >> 1);
+      } else {
+        c >>= 1;
+      }
+    }
+    kCrc32Table[i] = c;
+  }
+}
+
+uint32_t UpdateCrc32(uint32_t start, const void* buf, size_t len) {
+  EnsureCrc32TableInited();
+
+  uint32_t c = start ^ 0xFFFFFFFF;
+  const uint8_t* u = static_cast<const uint8_t*>(buf);
+  for (size_t i = 0; i < len; ++i) {
+    c = kCrc32Table[(c ^ u[i]) & 0xFF] ^ (c >> 8);
+  }
+  return c ^ 0xFFFFFFFF;
+}
+
+}  // namespace rtc
+