Clear away any lingering error before testing SkError.

tests/ErrorTest.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkError.h"
 #include "SkPath.h"
 #include "SkRect.h"
@@ skipping 10 matching lines @@
   {                                                                           \
      SkClearLastError();                                                      \
   }
 
 static void cb(SkError err, void *context) {
     ErrorContext *context_ptr = static_cast<ErrorContext *>(context);
     REPORTER_ASSERT( context_ptr->fReporter, (*(context_ptr->fIntPointer) == 0xdeadbeef) );
 }
 
 DEF_TEST(Error, reporter) {
+    // Some previous user of this thread may have left an error laying around.
+    SkClearLastError();
+
     SkError err;
 
     unsigned int test_value = 0xdeadbeef;
     ErrorContext context;
     context.fReporter = reporter;
     context.fIntPointer = &test_value;
 
     SkSetErrorCallback(cb, &context);
 
     CHECK(kNoError_SkError);
@@ skipping 11 matching lines @@
     // should trigger the default error callback, which just prints to the screen.
     path.addRoundRect(r, -10, -10);
     CHECK(kInvalidArgument_SkError);
     CHECK(kNoError_SkError);
 
     // should trigger *our* callback.
     path.addRoundRect(r, -10, -10);
     CHECK(kInvalidArgument_SkError);
     CHECK(kNoError_SkError);
 }