Remove some unneeded virtual inheritance.

All these classes have at most one subclass, so there's no point in
them being virtually derived.

magnification_controller.cc: Added in
https://chromiumcodereview.appspot.com/10388141 , the virtual
never had any effect.

The two download_files: Added in
https://codereview.chromium.org/8372034, never had any effect.

plugin_power_saver_browsertest: Added in
https://codereview.chromium.org/1088763002 , likely copied from
prerender_browsertest, never had any effect. (prerender_browsertest
grew it in  https://chromiumcodereview.appspot.com/10701140 and
it's sadly needed there.)
No intended behavior change.

BUG=none
TBR=sky

Review URL: https://codereview.chromium.org/1234883006

Cr-Commit-Position: refs/heads/master@{#339368}
diff --git a/content/browser/download/mock_download_file.h b/content/browser/download/mock_download_file.h
index 5d4b270..8be2c93 100644
--- a/content/browser/download/mock_download_file.h
+++ b/content/browser/download/mock_download_file.h
@@ -18,7 +18,7 @@
 namespace content {
 struct DownloadCreateInfo;
 
-class MockDownloadFile : virtual public DownloadFile {
+class MockDownloadFile : public DownloadFile {
  public:
   MockDownloadFile();
   virtual ~MockDownloadFile();