Re-enable LLVM LTO for ARM.

build/toolchain.gypi

Index: build/toolchain.gypi
diff --git a/build/toolchain.gypi b/build/toolchain.gypi
index a8a3b56ec22a2b761cfff51f95dc393b38000649..2163171d7e55a01f6d79beaa489a87f7f0175a58 100644
--- a/build/toolchain.gypi
+++ b/build/toolchain.gypi
@@ -277,10 +277,12 @@
                   }],
                 ],
               }],
-              # Disable LTO for v8
-              # v8 is optimized for speed, which takes precedence over
-              # size optimization in LTO.
-              ['use_lto==1', {
+              # Disable GCC LTO for v8
+              # v8 is optimized for speed. Because GCC LTO merges flags at link
+              # time, we disable LTO to prevent any -O2 flags from taking
+              # precedence over v8's -Os flag. However, LLVM LTO does not work
+              # this way so we keep LTO enabled under LLVM.
+              ['clang==0 and use_lto==1', {
                 'cflags!': [
                   '-flto',
                   '-ffat-lto-objects',