Fix scrollbar issue for Assistant opt-in screen

Update a few styles to fix the scrollbar issue on the value prop and
voice match screens.

Bug: b/143266371
Test: Manual Test
Change-Id: I147b81315b2d79cb879754b834d0f707256e5ea5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1880668Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Yue Li <updowndota@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709632}

chrome/browser/resources/chromeos/assistant_optin/assistant_shared_styles.css

@@ -51,6 +51,7 @@ oobe-dialog {
 #logo {
   height: 32px;
   margin: 0;
+  width: 32px;
 }
 
 #next-button {

chrome/browser/resources/chromeos/assistant_optin/assistant_value_prop.css

@@ -4,7 +4,7 @@
 
 #value-prop-view {
   display: block;
-  height: 256px;
+  height: 232px;
   margin: auto;
 }
 

chrome/browser/resources/chromeos/assistant_optin/assistant_voice_match.css

@@ -44,9 +44,10 @@
 #recording-container,
 #already-setup-container {
   display: none;
-  height: 460px;
+  flex-grow: 1;
 }
 
+.container,
 .intro #intro-container,
 .already-setup #already-setup-container {
   display: flex;