Annotate PageAllocator for MSan.

Annotate PageAllocator for MSan.

PageAllocator maps memory via sys_mmap(), implemented in
linux_syscall_support.h. We need to explicitly inform MSan that this memory is
initialized.

BUG=chromium:394028
R=mark@chromium.org
TEST=compile

[earthdok]

please take a look

[Mark Mentovai]

LGTM

[earthdok]

On 2014/07/22 12:32:27, Mark Mentovai wrote:
> LGTM

Could you please land this?

[Mark Mentovai]

Committed Breakpad r1356.

[earthdok]

On 2014/07/22 14:24:38, Mark Mentovai wrote:
> Committed Breakpad r1356.

Thanks!

[earthdok]

For the record:

this did not fix the spurious MSan report. It was actually caused by the fact
that components/breakpad/app/breakpad_linux.cc passes to PageAllocator::Alloc()
a |bytes| argument which is obtained from sys_fstat(). (And thus |bytes| is
poisoned when we check it in Alloc()).  Alas, I couldn't reproduce the issue on
my machine so there was no way to verify the fix.

As for whether this CL is actually necessary, it depends on whether there is any
code at all that makes the assumption that those particular mmap calls zero-fill
the memory. Users of this allocator probably shouldn't make that assumption (at
least it's not documented in the interface), but the allocator's implementation
could conceivably do that. I'm going to err on the side of caution and keep
those annotations.