serial: Fix break signal name for manual tests

The 'brk' signal was renamed to 'break' in r797507 however this manual
test was not updated.

Bug: 884928
Change-Id: I144e02e6bcca632a6ae4a72fbf31cfb7ba2dcf31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2405893
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Reviewed-by: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807035}

third_party/blink/web_tests/wpt_internal/serial/serialPort_loopback-manual.https.html

@@ -159,9 +159,9 @@
           }
         })();
 
-        await port.setSignals({brk: true});
+        await port.setSignals({break: true});
         await readPromise;
-        await port.setSignals({brk: false});
+        await port.setSignals({break: false});
 
         const writer = port.writable.getWriter();
         // |data| is small enough to be completely transmitted.