Remove remnants of old PCH code.

Remove the precompiled.cpp file which was producing a build warning,
and rename the header to make it clear we no longer support PCH.

BUG=angle:

Change-Id: I944081ae477b1478ca9c18924fb02bb920d420a7
Reviewed-on: https://chromium-review.googlesource.com/214187
Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libGLESv2/Context.cpp b/src/libGLESv2/Context.cpp
index 5cdc261..21b37b6 100644
--- a/src/libGLESv2/Context.cpp
+++ b/src/libGLESv2/Context.cpp
@@ -1,4 +1,3 @@
-#include "precompiled.h"
 //
 // Copyright (c) 2002-2014 The ANGLE Project Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
@@ -8,6 +7,7 @@
 // Context.cpp: Implements the gl::Context class, managing all GL state and performing
 // rendering operations. It is the GLES2 specific implementation of EGLContext.
 
+#include "libGLESv2/common_includes.h"
 #include "libGLESv2/Context.h"
 
 #include "libGLESv2/main.h"