Commit protoc 3.9.1 prebuilts from github.

These protoc prebuilts match the source code of external/protobuf.

Bug: 157512302
Change-Id: Ic07e67911a27262ef7b3b398d1b1493f44ec5ebd
diff --git a/README.txt b/README.txt
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+This project contains prebuilt binaries of the protocol buffer compiler.
+
+protoc is tightly coupled to the source code of libprotobuf-cpp, which is compiled from the sources in external/protobuf.
+
+Therefore, the contents of this folder should match the contents of external/protobuf.
+
+Download source: official protoc artifacts
+URL: https://github.com/protocolbuffers/protobuf/releases/tag/v3.9.1
diff --git a/darwin-x86_64/protoc b/darwin-x86_64/protoc
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diff --git a/linux-x86_64/protoc b/linux-x86_64/protoc
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diff --git a/windows-x86/protoc.exe b/windows-x86/protoc.exe
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+++ b/windows-x86/protoc.exe
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