Remove unused object field that broke some builds

It's not allowed to have unused object fields but the compiler
can't always know if a field is unused. In jumbo builds it will
know more so sometimes those detect cases that normal compilation
won't see. In this case a field only used in ChromeOS builds.

In file included from gen/chrome/browser/browser_jumbo_1.cc:80:
In file included from ./../../chrome/browser/chrome_content_browser_client.cc:428:
In file included from ../../chrome/browser/speech/extension_api/tts_engine_delegate_factory_impl.h:10:
../../chrome/browser/speech/extension_api/tts_engine_delegate_impl.h:35:28: error: private field 'browser_context_' is not used [-Werror,-Wunused-private-field]

  content::BrowserContext* browser_context_;
                           ^

TBR=dmazzoni@chromium.org

Bug: 862710
Change-Id: I4eca7b940ad49fb44ec61ec513ba54db55f4914c
Reviewed-on: https://chromium-review.googlesource.com/1225759
Commit-Queue: Daniel Bratell <bratell@opera.com>
Reviewed-by: Daniel Bratell <bratell@opera.com>
Reviewed-by: Jinho Bang <jinho.bang@samsung.com>
Cr-Commit-Position: refs/heads/master@{#591337}
2 files changed