Zero AudioBus internal aligned memory

This prevents from reading uninitialized memory according to MSAN
reports.

Bug: 856537
Change-Id: I750447942c3c0a41f49375cd1335068c4e5e34ce
Reviewed-on: https://chromium-review.googlesource.com/1116220
Commit-Queue: Dale Curtis <dalecurtis@chromium.org>
Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571932}

services/audio/output_controller_unittest.cc

@@ -347,6 +347,7 @@ class OutputControllerTest : public ::testing::Test {
         .WillRepeatedly(Return());
     EXPECT_CALL(mock_sync_reader_, Read(_))
         .WillOnce(Invoke([barrier](AudioBus* data) {
+          data->Zero();
           data->channel(0)[0] = kBufferNonZeroData;
           barrier.Run();
         }))