MD Bookmarks: Fix sidebar min/max width and Add dialog text

* Fixes an issue from crrev.com/467567, where it was possible to resize the
  sidebar without any limits on minimum or maximum width.
* Fixes an issue from crrev.com/467582, where text was colored white
  incorrectly in the bookmark add dialog.

BUG=692844
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2850673003
Cr-Commit-Position: refs/heads/master@{#467900}

chrome/browser/resources/md_bookmarks/app.html

@@ -36,19 +36,16 @@
         cursor: w-resize;
       }
 
-      bookmarks-sidebar {
-        max-width: 40%;
-        min-width: var(--min-sidebar-width);
-        width: var(--min-sidebar-width);
-      }
-
       #sidebar {
         -webkit-padding-start: 3px;
         display: block;
+        max-width: 40%;
+        min-width: var(--min-sidebar-width);
         overflow-x: hidden;
         overflow-y: auto;
         padding-top: 8px;
         user-select: none;
+        width: var(--min-sidebar-width);
       }
 
       bookmarks-list {

chrome/browser/resources/md_bookmarks/toolbar.html

@@ -13,7 +13,6 @@
     <style include="shared-style">
       :host {
         background: var(--md-toolbar-color);
-        color: #fff;
         transition: background-color 150ms;
       }
 
@@ -31,6 +30,7 @@
 
       cr-toolbar,
       cr-toolbar-selection-overlay {
+        color: #fff;
         flex: 1;
       }