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There are 2 different path that can possibly lead to
SetDataReductionProxyEnalbed:
1. AwContents initialization -> InitDataReductionProxyIfNecessary path
2. AwContents statistics SetDataReductionProxyEnalbed JNI path

In both paths, we need to make sure before SetDataReductionProxyEnabed
is called, we have set the statistics object on
DataReductionProxySettings.

BUG=b/17998706

Review URL: https://codereview.chromium.org/671823002

Cr-Commit-Position: refs/heads/master@{#300730}