Unify supersizer/index.html

Replace the two-column view for milestone vs canary builds with a one-column
view with a toggle for selecting milestone vs canary comparisons.

Also enforce certain conditions (mainly: diff-against version must
precede original version)

Bug: 1001775
Change-Id: Ie2f9b61b93e839898968d586b364883cbfa6c116
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1807070Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Jasper Chapman-Black <jaspercb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#697211}

tools/binary_size/libsupersize/static/options.css

@@ -238,11 +238,9 @@ select.sel-big:focus + .select-label::after {
 /** <input type='checkbox' or 'radio'> elements */
 input[type='checkbox'],
 input[type='radio'] {
-  position: absolute;
-  margin: 0;
+  margin: 4px 0;
   height: 18px;
   width: 18px;
-  opacity: 0;
 }
 .checkbox-wrapper,
 .radio-wrapper {