Revert "Enable frame pointers on arm32."

This reverts commit 8c29b757.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> Enable frame pointers on arm32.
> 
> LLVM fixed the bug where framepointers were not located next to the return
> address in thumb mode. https://bugs.llvm.org/show_bug.cgi?id=18505
> 
> With that fix, enabling framepointers on arm32 bring it in line with other
> build configurations and allows for better stack traces.
> 
> Bug: 786450
> Change-Id: I01a1c9f82fac22b2bbc15c6f52deed8aca7263ea
> Reviewed-on: https://chromium-review.googlesource.com/826255
> Reviewed-by: Brett Wilson <brettw@chromium.org>
> Commit-Queue: Albert J. Wong <ajwong@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#524127}

TBR=ajwong@chromium.org,brettw@chromium.org

Change-Id: Ie327529394021473cfffd0ee80baecce4bd83e2a
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 786450
Reviewed-on: https://chromium-review.googlesource.com/828607Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Albert J. Wong <ajwong@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524337}

build/config/compiler/compiler.gni

@@ -91,7 +91,7 @@ optimize_for_size = is_android || is_ios
 
 # Determine whether to enable or disable frame pointers, based on the platform
 # and build arguments.
-if (is_mac || is_ios || is_linux || is_chromeos) {
+if (is_mac || is_ios || is_linux) {
   enable_frame_pointers = true
 } else if (is_win) {
   # 64-bit Windows ABI doesn't support frame pointers.
@@ -100,10 +100,16 @@ if (is_mac || is_ios || is_linux || is_chromeos) {
   } else {
     enable_frame_pointers = true
   }
+} else if (is_chromeos) {
+  # 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 (is_android) {
   enable_frame_pointers =
       # Ensure that stacks from arm64 crash dumps are usable (crbug.com/391706).
-      current_cpu == "arm64" || current_cpu == "arm" ||
+      current_cpu == "arm64" ||
       # For x86 Android, unwind tables are huge without frame pointers
       # (crbug.com/762629). Enabling frame pointers grows the code size slightly
       # but overall shrinks binaries considerably by avoiding huge unwind
@@ -121,7 +127,11 @@ if (is_mac || is_ios || is_linux || is_chromeos) {
 # most translation units, that they are emitted correctly, and that the
 # compiler or platform provides a way to access them.
 can_unwind_with_frame_pointers = enable_frame_pointers
-if (is_win) {
+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
   # provide intrinsics to access them, so we don't use them.
   can_unwind_with_frame_pointers = false

tools/gn/bootstrap/bootstrap.py

@@ -185,6 +185,7 @@ def build_gn_with_ninja_manually(tempdir, options):
       {
           'ENABLE_LOCATION_SOURCE': 'false',
           'ENABLE_PROFILING': 'false',
+          'CAN_UNWIND_WITH_FRAME_POINTERS': 'false',
           'UNSAFE_DEVELOPER_BUILD': 'false'
       })