Remove legacy mailbox fallback on all platforms

Hopefully no longer needed on all platforms after r832898

Bug: 1153376
Change-Id: I7bc173cd4f404d6ababbe3e549f495b04bb028d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2571151Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833282}

components/viz/service/display_embedder/image_context_impl.cc

@@ -6,7 +6,6 @@
 
 #include <utility>
 
-#include "build/build_config.h"
 #include "components/viz/common/resources/resource_format_utils.h"
 #include "gpu/command_buffer/common/shared_image_usage.h"
 #include "gpu/command_buffer/service/mailbox_manager.h"
@@ -104,19 +103,12 @@ void ImageContextImpl::BeginAccessIfNecessary(
     std::vector<GrBackendSemaphore>* end_semaphores) {
   // Prepare for accessing shared image.
   if (mailbox_holder().mailbox.IsSharedImage()) {
-    constexpr bool allow_legacy_mailbox_fallback =
-#if defined(OS_ANDROID)
-        false;
-#else
-        true;
-#endif
-    bool result = BeginAccessIfNecessaryForSharedImage(
-        context_state, representation_factory, begin_semaphores,
-        end_semaphores);
-    if (!result && !allow_legacy_mailbox_fallback) {
+    if (!BeginAccessIfNecessaryForSharedImage(
+            context_state, representation_factory, begin_semaphores,
+            end_semaphores)) {
       CreateFallbackImage(context_state);
-      return;
     }
+    return;
   }
 
   // Prepare for accessing legacy mailbox.