[SuperSize] Fix BaseSizeInfo.ContainerForName().

|container| should be |self.container|. This causes Diff() to fail in
SuperSize-console.

TBR=agrieve@chromium.org

Bug: 1040645
Change-Id: Ie82e63f6449b905e628ebf884fd157c603dc6bf7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2264753
Reviewed-by: Samuel Huang <huangs@chromium.org>
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781937}

tools/binary_size/libsupersize/models.py

@@ -319,7 +319,7 @@ def metadata(self):
     return [c.metadata for c in self.containers]
 
   def ContainerForName(self, name, default=None):
-    return next((c for c in containers if c.name == name), default)
+    return next((c for c in self.containers if c.name == name), default)
 
 
 class SizeInfo(BaseSizeInfo):