Revert "mac and ios: Turn linker warnings into errors."

This reverts commit add19d06.

Reason for revert:

Findit (https://goo.gl/kROfz5) identified CL at revision 681434 as the
culprit for failures in the build cycles as shown on:
https://analysis.chromium.org/waterfall/culprit?key=ag9zfmZpbmRpdC1mb3ItbWVyRAsSDVdmU3VzcGVjdGVkQ0wiMWNocm9taXVtL2FkZDE5ZDA2YjQ3YzgxNTFkYWM2YWFiMzU4NWU1NTJhMDQ3ZDE4OWIM

Sample Failed Build: https://ci.chromium.org/buildbot/chromium/mac-archive-rel/2785

Sample Failed Step: compile

Original change's description:
> mac and ios: Turn linker warnings into errors.
> 
> Bug: 747727
> Change-Id: I33f44e3f4011873e193088796483ecdc48f7f05e
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720610
> Reviewed-by: Mark Mentovai <mark@chromium.org>
> Commit-Queue: Nico Weber <thakis@chromium.org>
> Auto-Submit: Nico Weber <thakis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#681434}


Change-Id: Icb3d15c4d46a3b4bbadf2f867fc3d542a375044e
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 747727
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1720743
Cr-Commit-Position: refs/heads/master@{#681463}

build/config/compiler/BUILD.gn

@@ -82,6 +82,7 @@ declare_args() {
 
   # Enable fatal linker warnings. Building Chromium with certain versions
   # of binutils can cause linker warning.
+  # See: https://bugs.chromium.org/p/chromium/issues/detail?id=457359
   fatal_linker_warnings = true
 
   # Build with C++ RTTI enabled. Chromium builds without RTTI by default,
@@ -300,9 +301,6 @@ config("compiler") {
       # crbug.com/485542
       ldflags += [ "-Wl,--fatal-warnings" ]
     }
-    if (fatal_linker_warnings && (is_mac || is_ios)) {
-      ldflags += [ "-Wl,-fatal_warnings" ]
-    }
   } else {
   }