cmd/gogio: log chrome's console messages in TestJSOnChrome

For example, if the browser doesn't have webgl at all, the gio app will
fail to load. This will result in the screenshot being incorrect,
without an apparent reason:

	--- FAIL: TestJSOnChrome (0.89s)
	    js_test.go:122: got 0xffffffffffffffff at (5,5), want 0xffff00000000ffff
	    js_test.go:122: got 0xffffffffffffffff at (595,595), want 0xffff00000000ffff

The underlying webgl error was accessible if one added a sleep and ran
'go test -headless=false', allowing to open the console and see the
error messages.

Instead, capture them via chromedp and print them to the test's logger:

	--- FAIL: TestJSOnChrome (0.89s)
	    js_test.go:79: console log: "2019/10/29 12:41:07 app: webgl is not supported"
	    js_test.go:79: console warning: "exit code:", 1
	    js_test.go:122: got 0xffffffffffffffff at (5,5), want 0xffff00000000ffff
	    js_test.go:122: got 0xffffffffffffffff at (595,595), want 0xffff00000000ffff

JS Exceptions are a completely different mechanism, so they're not
covered by this patch. We can add them at a later time if needed.

While at it, update to the latest tagged version of chromedp.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>

cmd/gogio/js_test.go

@@ -13,8 +13,10 @@ import (
 	"net/http/httptest"
 	"os"
 	"os/exec"
+	"strings"
 	"testing"
 
+	"github.com/chromedp/cdproto/runtime"
 	"github.com/chromedp/chromedp"
 
 	_ "gioui.org/unit" // the build tool adds it to go.mod, so keep it there
@@ -62,6 +64,22 @@ func TestJSOnChrome(t *testing.T) {
 		}
 		t.Fatal(err)
 	}
+	chromedp.ListenTarget(ctx, func(ev interface{}) {
+		switch ev := ev.(type) {
+		case *runtime.EventConsoleAPICalled:
+			switch ev.Type {
+			case "log", "info", "warning", "error":
+				var args strings.Builder
+				for i, arg := range ev.Args {
+					if i > 0 {
+						args.WriteString(", ")
+					}
+					args.Write(arg.Value)
+				}
+				t.Logf("console %s: %s", ev.Type, args.String())
+			}
+		}
+	})
 
 	// Third, serve the app folder, set the browser tab dimensions, and
 	// navigate to the folder.