[Profile creation] Align size of the WebUI back button with mocks

This CL aligns the size of the WebUI back button with the mocks and thus
also with the size of the native UI back button.

Bug: 1126913
Change-Id: I72857d1a9cf870f75f0e0f16bd58f75544063c55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577362
Commit-Queue: Jan Krcal <jkrcal@chromium.org>
Commit-Queue: Monica Basta <msalama@chromium.org>
Auto-Submit: Jan Krcal <jkrcal@chromium.org>
Reviewed-by: Monica Basta <msalama@chromium.org>
Cr-Commit-Position: refs/heads/master@{#834269}

chrome/browser/resources/signin/profile_picker/profile_creation_flow/shared_css.html

@@ -23,7 +23,7 @@
 
     #backButton {
       --cr-icon-button-fill-color: var(--theme-text-color);
-      --cr-icon-button-icon-size: 20px;
+      --cr-icon-button-icon-size: 22px;
       --cr-icon-button-margin-start: 4px;
       --cr-icon-button-size: 36px;
       margin-top: 4px;