[ios] Fixes a compile error under ASAN.

A new method was added but never called in ASAN builds, causing an
unused function warning.

BUG=None

Change-Id: Ibca08fb8ab291790fcb58f617844bc6a0730c8fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2341807Reviewed-by: Eugene But <eugenebut@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#795983}

ios/chrome/browser/ios_chrome_main_parts.mm

@@ -77,6 +77,8 @@
 #endif
 
 namespace {
+
+#if BUILDFLAG(USE_ALLOCATOR_SHIM)
 // Do not install allocator shim on iOS 13.4 due to high crash volume on this
 // particular version of OS. TODO(crbug.com/1108219): Remove this workaround
 // when/if the bug gets fixed.
@@ -84,6 +86,8 @@ bool ShouldInstallAllocatorShim() {
   return !base::ios::IsRunningOnOrLater(13, 4, 0) ||
          base::ios::IsRunningOnOrLater(13, 5, 0);
 }
+#endif
+
 }  // namespace
 
 IOSChromeMainParts::IOSChromeMainParts(