Cleanup: Remove MD5_CTX typedef.

Instead just use MD5Context type directly. In C++ it is unnecessary to
alias the types using typedef, unline C (where if you don't you have to
spell out struct or enum infront of the user-type everytime you want to make a
variable).

So since WebRTC's base API is C++, it seems unnecessay to keep this
typedef around.

BUG=None
TEST=rtc_unittests --gtest_filter=Md5*
R=tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/46799004

Patch from Thiago Farina <tfarina@chromium.org>.

Cr-Commit-Position: refs/heads/master@{#8916}

webrtc/base/md5.h

@@ -26,9 +26,6 @@
 
 namespace rtc {
 
-// Canonical name for a MD5 context structure, used in many crypto libs.
-typedef struct MD5Context MD5_CTX;
-
 struct MD5Context {
   uint32 buf[4];
   uint32 bits[2];

webrtc/base/md5digest.h

@@ -28,7 +28,7 @@ class Md5Digest : public MessageDigest {
   size_t Finish(void* buf, size_t len) override;
 
  private:
-  MD5_CTX ctx_;
+  MD5Context ctx_;
 };
 
 }  // namespace rtc