Fix build rules for gRPC proto generator plugin to work when crosscompiling

Fix build rules for gRPC proto generator plugin to work when crosscompiling

The protoc compiler is only available for host toolchain, not for target
toolchain. Therefore we can only have targets depending on it use host
toolchain too.

BUG=618889
R=marcinjb@chromium.org

Committed: https://chromium.googlesource.com/external/github.com/grpc/grpc/+/dc44fe515c8c30093ea8b2f14230123959c886e8

[xyzzyz, 2016-08-19 00:12:30]

xyzzyz@chromium.org changed reviewers:
+ gcasto@chromium.org, marcinjb@chromium.org

[xyzzyz, 2016-08-19 00:12:31]

PTAL. The BUILD.gn file is not indented properly, but it's either this, or
making the template file terrible, disgusting and generally double-minus-bad.

[marcinjb, 2016-08-19 00:16:07]

lgtm

[xyzzyz, 2016-08-19 00:21:57]

Description was changed from

==========
Fix build rules for gRPC proto generator plugin to work when crosscompiling

The protoc compiler is only available for host toolchain, not for target
toolchain. Therefore we can only have targets depending on it use host
toolchain too.

BUG=618889
==========

to

==========
Fix build rules for gRPC proto generator plugin to work when crosscompiling

The protoc compiler is only available for host toolchain, not for target
toolchain. Therefore we can only have targets depending on it use host
toolchain too.

BUG=618889
R=marcinjb@chromium.org

Committed:
https://chromium.googlesource.com/external/github.com/grpc/grpc/+/dc44fe515c8...
==========

[xyzzyz, 2016-08-19 00:21:58]

Committed patchset #1 (id:1) manually as
dc44fe515c8c30093ea8b2f14230123959c886e8 (presubmit successful).