[mojo-edk] Initialize message buffers after alloc

We weren't zeroing the allocated memory before filling
the buffer. Due to padding this means there could be
uninitialized bytes. Bad news. Fixed.

Should fix failures on memory bots:

https://build.chromium.org/p/chromium.memory.fyi/builders/Windows%20Content%20Browser%20%28DrMemory%20full%29%20%284%29/builds/7154

BUG=None
TBR=thestig@chromium.org,darin@chromium.org

Review URL: https://codereview.chromium.org/1641493002

Cr-Commit-Position: refs/heads/master@{#371737}

mojo/edk/system/channel.cc

@@ -36,6 +36,7 @@ Channel::Message::Message(size_t payload_size, size_t num_handles) {
 
   data_ = static_cast<char*>(base::AlignedAlloc(size_,
                                                 kChannelMessageAlignment));
+  memset(data_, 0, size_);
   header_ = reinterpret_cast<Header*>(data_);
 
   DCHECK_LE(size_, std::numeric_limits<uint32_t>::max());