Clear bitmap from canvas after done drawing to it.

Change-Id: I0ebc8510021a2dd0b147ced7890d939ec47809f6
diff --git a/src/com/android/browser/Tab.java b/src/com/android/browser/Tab.java
index cd209ed..6aa8683 100644
--- a/src/com/android/browser/Tab.java
+++ b/src/com/android/browser/Tab.java
@@ -1993,6 +1993,7 @@
         float scale = mCaptureWidth / (float) mMainView.getWidth();
         c.scale(scale, scale, left, top);
         mMainView.draw(c);
+        c.setBitmap(null);
         persistThumbnail();
     }