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Make the developer mode checkbox on chrome://extensions adhere to the DeveloperToolsDisabled policy. If it is disabled, display an indicator.
Add corresponding indicator test to policy_prefs_browsertest.
Add an end-to-end test to policy_browsertest.

BUG=633684
TEST=browser_tests *PolicyPrefIndicatorTest*, PolicyTest.DeveloperToolsDisabledExtensionsDevMode
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:closure_compilation

Review-Url: https://codereview.chromium.org/2529083002
Cr-Commit-Position: refs/heads/master@{#438215}