Use LIKELY and UNLIKELY macros to wrap error generation.

BUG=609673

Change-Id: I247f561712e30aa10bad77fabf614501d596915e
Reviewed-on: https://chromium-review.googlesource.com/1021995
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>
diff --git a/src/libANGLE/Context.cpp b/src/libANGLE/Context.cpp
index d081595..2a24712 100644
--- a/src/libANGLE/Context.cpp
+++ b/src/libANGLE/Context.cpp
@@ -2453,7 +2453,7 @@
 
 void Context::handleError(const Error &error) const
 {
-    if (error.isError())
+    if (ANGLE_UNLIKELY(error.isError()))
     {
         GLenum code = error.getCode();
         mErrors.insert(code);