Mark battery-status/battery-full-manual.https.html as flaky

It was marked as failing, but
https://crrev.com/b71bef1cb23070af161d251d9188ea153bed67d8 removed the
expectation. Since the flakiness persists, let's recover the
expectation.

Bug: 833100
Change-Id: If5f3d61e9cebb3fa9542ddbcf5f971a67340f8f6
No-Try: True
Tbr: timvolodine@chromium.org
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1939137Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#719526}

third_party/blink/web_tests/TestExpectations

@@ -5899,3 +5899,5 @@ crbug.com/1028116 virtual/gpu-rasterization/images/webp-color-profile-lossy-alph
 crbug.com/1028116 virtual/gpu-rasterization/images/yuv-decode-eligible/color-profile-filter.html [ Pass Failure ]
 crbug.com/1028116 virtual/gpu-rasterization/images/yuv-decode-eligible/color-profile-image.html [ Pass Failure ]
 crbug.com/1028116 virtual/gpu-rasterization/images/yuv-decode-eligible/webp-color-profile-lossy.html [ Pass Failure ]
+
+crbug.com/833100 [ Mac ] external/wpt/battery-status/battery-full-manual.https.html [ Pass Failure ]