[Memory] Fix OS::Allocate on Windows / Cygwin.

- Changes assert to reflect that we might exceed the number of attempts
  to allocate a padded memory region while attempting to get an aligned
  allocation and return null instead.

Bug: chromium:813587
Change-Id: I0e225f50b7a64a2f2a72de15322525574445efbd
Reviewed-on: https://chromium-review.googlesource.com/926968Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Bill Budge <bbudge@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51401}

src/base/platform/platform-cygwin.cc

@@ -139,7 +139,7 @@ void* OS::Allocate(void* address, size_t size, size_t alignment,
     // base will be nullptr.
     if (base != nullptr) break;
   }
-  DCHECK_EQ(base, aligned_base);
+  DCHECK_IMPLIES(base, base == aligned_base);
   return reinterpret_cast<void*>(base);
 }
 

src/base/platform/platform-win32.cc

@@ -829,7 +829,7 @@ void* OS::Allocate(void* address, size_t size, size_t alignment,
     // base will be nullptr.
     if (base != nullptr) break;
   }
-  DCHECK_EQ(base, aligned_base);
+  DCHECK_IMPLIES(base, base == aligned_base);
   return reinterpret_cast<void*>(base);
 }