Fix Files App progress-center scrolling.

The old scrolling scrolled the entire footer; since the
progress-center-open-view is always present, but below the fold, this
allowed the progress-center-open-view to be scrolled off the top of the
footer element. Avoid this by only allowing scrolling on the
progress-center-open-view element. Additionally, restrict its height to
40% of the window so the directory tree can remain usable while the
progress center is expanded.

Slightly improve the position of notification dismiss buttons: align
them to flex-start instead of flex-end and use display: none for the
cancel button when hidden.

Bug: 627371
No-try: true
Change-Id: I2180eb8382c37c1fc03ec32b4894f449f4c7a681
Reviewed-on: https://chromium-review.googlesource.com/c/1351202
Commit-Queue: Sam McNally <sammc@chromium.org>
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Cr-Commit-Position: refs/heads/master@{#611068}

ui/file_manager/file_manager/foreground/css/file_manager.css

@@ -104,9 +104,8 @@ a:focus {
 
 .dialog-navigation-list-footer {
   display: flex;
-  flex: 0 1 auto;
+  flex: none;
   flex-direction: column;
-  overflow: auto;
 }
 
 /* A vertical splitter between the roots list and the file list. It is actually
@@ -2091,7 +2090,9 @@ list.autocomplete-suggestions > [lead] {
 }
 
 #progress-center-open-view {
+  max-height: 40vh;
   opacity: 1;
+  overflow-y: auto;
   padding-top: 10px;
   transition: opacity 300ms linear;
 }
@@ -2177,7 +2178,7 @@ list.autocomplete-suggestions > [lead] {
 }
 
 #progress-center .button-frame {
-  align-self: flex-end;
+  align-self: flex-start;
   flex: none;
 }
 
@@ -2251,7 +2252,7 @@ li#progress-center-close-view.error.single button.dismiss,
 
 #progress-center-close-view.quiet button.cancel,
 #progress-center li:not(.cancelable) button.cancel {
-  visibility: hidden;
+  display: none;
 }
 
 .text-measure {