Build with enable_webvr=0 by default.
Build with enable_webvr=0 by default.

*IMPORTANT*
We actually intend to fix this properly. We are cherry-picking this
commit from the crosswalk-18 branch first in order to have a stable
baseline with no WebVR support propagating from Crosswalk 21 to 20 to
19. After that, we can start working on fixing this in 21 and 20 and, if
there is enough time, 19 too.

From the original commit:

Disable WebVR support on Android by default, since it depends on
`third_party/cardboard-java` which in turn needs
`third_party/android_protobuf`.

The dependency on `android_protobuf` means `xwalk_core_library_java.jar`
bundle Protobuf and cause build conflicts for users depending on
Protobuf themselves (or using other libraries that pull it).

BUG=XWALK-6597

(cherry picked from commit 5666bef3d9784799aefa34bbccfcb4c187825d45)

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