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The original CL is here: https://codereview.chromium.org/2367673002/
It was reverted because of compile failure with gn flags:
is_debug=false
is_component_build=true
target_os="android"
The reason is gvr uses API that only available in Android N SDK. For
release build, proguard detected the dependancy and results a compile
error. N SDK is in the process of rolling to chromium
https://codereview.chromium.org/2350013004/
In this CL, I restricted WebVr to debug build only. This could unblock
us. And I will remove the is_debug restriction once N is in Chromium

BUG=389343
TBR=dtrainor@

Review-Url: https://codereview.chromium.org/2396663002
Cr-Commit-Position: refs/heads/master@{#423005}