Clear away any lingering error before testing SkError.

Looks like something's setting an error before ErrorTest runs on our new
Venue8 bots.

BUG=skia:
R=humper@google.com, mtklein@google.com

Author: mtklein@chromium.org

Review URL: https://codereview.chromium.org/551973004

tests/ErrorTest.cpp

@@ -28,6 +28,9 @@ static void cb(SkError err, void *context) {
 }
 
 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;