fuchsia: Make sender_pipe_unittest use the posix version

I'm not sure if this will be correct until these tests can run, but this
will at least compile, as opposed to using the Windows version.

Bug: 808287
Change-Id: I2f112f9ab2575abc88d1495ba324cdcc15cde4a7
Reviewed-on: https://chromium-review.googlesource.com/c/1298299Reviewed-by: Erik Chen <erikchen@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#602475}

components/services/heap_profiling/public/cpp/sender_pipe_unittest.cc

@@ -34,7 +34,7 @@ class SenderPipeTest : public testing::Test {
   Result Write(int size) { return sender_pipe_->Send(buffer_.data(), size, 1); }
 
   void Read(int size) {
-#if defined(OS_POSIX)
+#if defined(OS_POSIX) || defined(OS_FUCHSIA)
     ssize_t bytes_read = read(read_handle_.GetFD().get(), buffer_.data(), size);
     ASSERT_EQ(size, bytes_read);
 #else