Re-enable LLVM LTO for ARM.

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.

This fixes the ARM build in the cfi_vptr==1 configuration, which requires
LLVM LTO.

R=adamk@chromium.org
BUG=chromium:469376
LOG=N

Review URL: https://codereview.chromium.org/1295673002

Cr-Commit-Position: refs/heads/master@{#30455}

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',