Always define ABSL_ALLOCATOR_NOTHROW=1.

From absl/memory/memory.h:
// ABSL_ALLOCATOR_NOTHROW is a build time configuration macro for user to
// specify whether the default allocation function can throw or never throws.
// If the allocation function never throws, user should define it to a non-zero
// value (e.g. via `-DABSL_ALLOCATOR_NOTHROW`).
// If the allocation function can throw, user should leave it undefined or
// define it to zero.

Since exceptions are not allowed in Chromium, it is probably safe to assume
the default allocation function also never throws.

Bug: webrtc:8821
Change-Id: I18e617ae75084f32016c1b030375c0b81ca9aa50
Reviewed-on: https://chromium-review.googlesource.com/1018103Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552082}

third_party/abseil-cpp/BUILD.gn

@@ -23,11 +23,7 @@ config("absl_include_config") {
 }
 
 config("absl_define_config") {
-  if (is_linux) {
-    cflags_cc = [ "-DABSL_ALLOCATOR_NOTHROW=1" ]
-  } else {
-    cflags_cc = [ "-DABSL_ALLOCATOR_NOTHROW=0" ]
-  }
+  defines = [ "ABSL_ALLOCATOR_NOTHROW=1" ]
 }
 
 config("absl_default_cflags_cc") {