Update Bazel's protobuf from 2.5.0 to 3.0.0-alpha3 for proto3 support.

--
MOS_MIGRATED_REVID=95639138
diff --git a/scripts/bootstrap/compile.sh b/scripts/bootstrap/compile.sh
index 9745a9c..826dd20 100755
--- a/scripts/bootstrap/compile.sh
+++ b/scripts/bootstrap/compile.sh
@@ -78,7 +78,7 @@
   MD5SUM="md5sum"
   # JAVA_HOME must point to a Java installation.
   JAVA_HOME="${JAVA_HOME:-$(readlink -f $(which javac) | sed 's_/bin/javac__')}"
-  PROTOC=${PROTOC:-third_party/protobuf/protoc.linux-i686}
+  PROTOC=${PROTOC:-third_party/protobuf/protoc-linux-x86_32.exe}
   ;;
 
 darwin)
@@ -89,7 +89,7 @@
     JAVA_HOME="$(/usr/libexec/java_home -v ${JAVA_VERSION}+ 2> /dev/null)" \
       || fail "Could not find JAVA_HOME, please ensure a JDK (version ${JAVA_VERSION}+) is installed."
   fi
-  PROTOC=${PROTOC:-third_party/protobuf/protoc.darwin}
+  PROTOC=${PROTOC:-third_party/protobuf/protoc-osx-x86_32.exe}
   ;;
 
 msys*|mingw*)
diff --git a/src/test/shell/bazel/testenv.sh b/src/test/shell/bazel/testenv.sh
index a275ad5..83dc339 100755
--- a/src/test/shell/bazel/testenv.sh
+++ b/src/test/shell/bazel/testenv.sh
@@ -43,10 +43,10 @@
 PLATFORM="$(uname -s | tr 'A-Z' 'a-z')"
 case "${PLATFORM}" in
   darwin)
-    protoc_compiler="${TEST_SRCDIR}/third_party/protobuf/protoc.darwin"
+    protoc_compiler="${TEST_SRCDIR}/third_party/protobuf/protoc-osx-x86_32.exe"
     ;;
   *)
-    protoc_compiler="${TEST_SRCDIR}/third_party/protobuf/protoc.linux-i686"
+    protoc_compiler="${TEST_SRCDIR}/third_party/protobuf/protoc-linux-x86_32.exe"
     ;;
 esac
 protoc_jar="${TEST_SRCDIR}/third_party/protobuf/protobuf-*.jar"
diff --git a/third_party/BUILD b/third_party/BUILD
index 186a6e0..146f63d 100644
--- a/third_party/BUILD
+++ b/third_party/BUILD
@@ -8,11 +8,11 @@
 filegroup(
     name = "protoc",
     srcs = select({
-        ":windows_mingw": ["protobuf/protoc.exe"],
-        ":windows_msys64_mingw64": ["protobuf/protoc.exe"],
-        ":windows_clang": ["protobuf/protoc.exe"],
-        ":darwin": ["protobuf/protoc.darwin"],
-        "//conditions:default": ["protobuf/protoc.linux-i686"],
+        ":windows_mingw": ["protobuf/protoc-windows-x86_32.exe"],
+        ":windows_msys64_mingw64": ["protobuf/protoc-windows-x86_64.exe"],
+        ":windows_clang": ["protobuf/protoc-windows-x86_32.exe"],
+        ":darwin": ["protobuf/protoc-osx-x86_32.exe"],
+        "//conditions:default": ["protobuf/protoc-linux-x86_32.exe"],
     }),
 )
 
@@ -224,13 +224,13 @@
 
 java_import(
     name = "protobuf",
-    jars = ["protobuf/protobuf-2.5.0.jar"],
+    jars = ["protobuf/protobuf-java-3.0.0-alpha-3.jar"],
 )
 
 # For bootstrapping JavaBuilder
 filegroup(
     name = "protobuf-jars",
-    srcs = ["protobuf/protobuf-2.5.0.jar"],
+    srcs = ["protobuf/protobuf-java-3.0.0-alpha-3.jar"],
 )
 
 java_import(