Use an html instead of a xul progress bar for better perf on Linux (bug 1003664). r=jlongster, f=darktrojan

browser/devtools/debugger/debugger.css

@@ -48,3 +48,13 @@
 #body[layout=vertical] #stackframes {
   visibility: hidden;
 }
+
+#source-progress-container {
+  display: flex;
+  flex-flow: column;
+  justify-content: center;
+}
+
+#source-progress {
+  flex: none;
+}

browser/devtools/debugger/debugger.xul

@@ -15,6 +15,7 @@
 <?xul-overlay href="chrome://global/content/editMenuOverlay.xul"?>
 
 <window xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
+        xmlns:html="http://www.w3.org/1999/xhtml"
         macanimationtype="document"
         fullscreenbutton="true"
         screenX="4" screenY="4"
@@ -421,12 +422,12 @@
                   label="&debuggerUI.blackBoxMessage.unBlackBoxButton;"
                   command="unBlackBoxCommand"/>
         </vbox>
-        <vbox id="source-progress-container"
-              align="center"
-              pack="center">
-          <progressmeter id="source-progress"
-                         mode="undetermined"/>
-        </vbox>
+        <html:div id="source-progress-container"
+                  align="center">
+          <html:div id="hbox">
+            <html:progress id="source-progress"></html:progress>
+          </html:div>
+        </html:div>
       </deck>
       <splitter id="editor-and-instruments-splitter"
                 class="devtools-side-splitter"/>