MD History: Fix position of Clear Browsing Data menu promo

The positioning of the menu promo in MD History was accidentally broken
in crrev.com/439710 when the layout of the left section of the toolbar
was changed.

BUG=677902
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2609933003
Cr-Commit-Position: refs/heads/master@{#441439}

chrome/browser/resources/md_downloads/vulcanized.html

@@ -3089,6 +3089,7 @@ h1 {
 
 #leftContent {
   -webkit-margin-start: 12px;
+        position: relative;
         transition: opacity 100ms;
 }
 

chrome/browser/resources/md_history/app.vulcanized.html

@@ -1957,6 +1957,7 @@ h1 {
 
 #leftContent {
   -webkit-margin-start: 12px;
+        position: relative;
         transition: opacity 100ms;
 }
 

ui/webui/resources/cr_elements/cr_toolbar/cr_toolbar.html

@@ -29,6 +29,7 @@
       #leftContent {
         /* margin-start here must match margin-end on #rightContent. */
         -webkit-margin-start: 12px;
+        position: relative;
         transition: opacity 100ms;
       }