scanning: Fix progress bar width

Make the progress bar width a percentage of the preview area width so
that it scales with the preview area. This resolves an issue where the
scan progress and canceling progress bars would extend outside of the
preview area at small app window sizes.

Bug: 1059779
Change-Id: I3d3f198a6a79f376d635f202ea84e5b7cdcd866e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2582805Reviewed-by: Zentaro Kavanagh <zentaro@chromium.org>
Commit-Queue: Jesse Schettler <jschettler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835386}

chromeos/components/scanning/resources/scan_preview.html

@@ -28,7 +28,7 @@
   #cancelingProgress {
     margin: auto;
     text-align: center;
-    width: 60%;
+    width: 70%;
   }
 
   .preview {
@@ -62,7 +62,7 @@
     border-radius: 4px;
     height: 4px;
     margin-top: 12px;
-    width: 256px;
+    width: 100%;
   }
 
   #scanProgressBar {