[web tests] results detail sizing fix

Image detail canvases sizes were different.
This forces all flex items to the same size.

Change-Id: I4db361c5b1a1746a882ea903dc142d3614cba02d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2401805Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Commit-Queue: Aleks Totic <atotic@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806328}

third_party/blink/web_tests/fast/harness/results.html

@@ -2229,6 +2229,7 @@ class ImageViewerZoom {
       position: relative;
       flex-grow: 1;
       flex-shrink: 1;
+      flex-basis: 100px;
       margin: 16px 8px 16px 8px;
     }
     .image-viewer-zoom-tile > canvas {