build: On Android, disable LTO for specific targets only when targeting GCC.

build/common.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 666 matching lines @@
 
       # Automatically select platforms under ozone. Turn this off to
       # build only explicitly selected platforms.
       'ozone_auto_platforms%': 1,
 
       # If this is set clang is used as host compiler, but not as target
       # compiler. Always do this by default.
       'host_clang%': 1,
 
       # Variables to control Link-Time Optimization (LTO).
-      # On Android, the variable use_lto enables LTO on code compiled with -Os,
-      # and use_lto_o2 enables LTO on code compiled with -O2. On other
-      # platforms, use_lto enables LTO in all translation units, and use_lto_o2
-      # has no effect.
+      # On Android, when using GCC LTO, the variable use_lto enables LTO on code
+      # compiled with -Os, and use_lto_o2 enables LTO on code compiled with -O2.
+      # On other platforms (including Android with Clang), use_lto enables LTO
+      # in all translation units, and use_lto_o2 has no effect.
       #
       # On Linux and Android, when using LLVM LTO, the script
       # build/download_gold_plugin.py must be run to download a linker plugin.
       # On Mac, LLVM needs to be built from scratch using
       # tools/clang/scripts/update.py and the absolute path to
       # third_party/llvm-build/Release+Asserts/lib must be added to
       # $DYLD_LIBRARY_PATH to pick up the right version of the linker plugin.
+      # TODO(pcc): Teach build system to use -lto_library flag to specify path
+      # to linker plugin on Mac.
       #
-      # On Android, the variables must *not* be enabled at the same time.
+      # On Android/GCC, the variables must *not* be enabled at the same time.
       # In this case LTO would 'merge' the optimization flags at link-time
       # which would lead to all code be optimized with -O2. See crbug.com/407544
       'use_lto%': 0,
       'use_lto_o2%': 0,
 
       # Allowed level of identical code folding in the gold linker.
       'gold_icf_level%': 'all',
 
       # Libxkbcommon usage.
       'use_xkbcommon%': 0,
@@ skipping 5606 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }