Fix signed/unsigned comparison in select_to_speak_browsertest.cc

This is causing my build to fail locally. I'm not sure why it's
working on the bots.

Bug: none
Change-Id: I3d715d9ce69637329719ea944da68c7ed5830a27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2633200Reviewed-by: Jun Mukai <mukai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#844336}

chrome/browser/ash/accessibility/select_to_speak_browsertest.cc

@@ -92,7 +92,7 @@ class SelectToSpeakTest : public InProcessBrowserTest {
 
   void TearDownOnMainThread() override {
     // Check STS has not generated any errors.
-    EXPECT_EQ(0, console_observer_->messages().size())
+    EXPECT_EQ(0u, console_observer_->messages().size())
         << "Found console.log or console.warn with message: "
         << console_observer_->GetMessageAt(0);
   }