Update the enable_frame_pointers logic for ChromeOS.

Now that we have a patch for the ARM A12/A17 errata, it is safe to try
building ARM Thumb without frame-pointers again, since we don't need
them.

We leave frame-pointers enabled in ChromeOS for 64-bit.

The Windows case is tweaked to use current_cpu rather than target_cpu,
and the Android case is explicitly gated on is_android.

Bug: 740806, 711784
Change-Id: I2fff46d1a3a8d78d12c389f95a20acf54013318d
Reviewed-on: https://chromium-review.googlesource.com/567608
Commit-Queue: Wez <wez@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486165}

build/config/compiler/compiler.gni

@@ -78,11 +78,11 @@ if (is_mac || is_ios) {
     enable_frame_pointers = true
   }
 } else if (is_chromeos) {
-  # ChromeOS requires frame pointers in x64 builds, to support CWP.
-  # TODO(711784): Building ARM Thumb without frame pointers can lead to code
-  # in ChromeOS which triggers some ARM A12/A17 errata. They can be disabled
-  # on non-x64 once that is resolved.
-  enable_frame_pointers = true
+  # ChromeOS generally prefers frame pointers, to support CWP.
+  # However, Clang does not currently generate usable frame pointers in ARM
+  # 32-bit builds (https://bugs.llvm.org/show_bug.cgi?id=18505) so disable them
+  # there to avoid the unnecessary overhead.
+  enable_frame_pointers = current_cpu != "arm"
 } else if (current_cpu == "arm64") {
   # Ensure that stacks from arm64 crash dumps are usable (crbug.com/391706).
   enable_frame_pointers = true
@@ -100,6 +100,7 @@ if (is_mac || is_ios) {
 can_unwind_with_frame_pointers = enable_frame_pointers
 if (current_cpu == "arm" && arm_use_thumb) {
   # We cannot currently unwind ARM Thumb frame pointers correctly.
+  # See https://bugs.llvm.org/show_bug.cgi?id=18505
   can_unwind_with_frame_pointers = false
 } else if (is_win) {
   # Windows 32-bit does provide frame pointers, but the compiler does not