Make exclude_unwind_tables into a GN arg again.

This was removed from the declare_args section some time ago, but for
AOSP system implementors it may be desirable to manually control it.
Re-add it as an argument, and document how to use it in the WebView AOSP
build guide.

Fixed: 1034192
Change-Id: Ic5f890f2ad3677bb76ce87951ee0d7d3f20551b9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1976331Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Commit-Queue: Richard Coles <torne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730188}

android_webview/docs/aosp-system-integration.md

@@ -195,10 +195,6 @@ is_component_build = false
 # Disable Google-specific branding/features
 is_chrome_branded = false
 use_official_google_api_keys = false
-
-# Significantly reduces binary size at the cost of preventing Android's native
-# crash handler from being able to produce useful stack unwindings.
-exclude_unwind_tables = true
 ```
 
 The `target_cpu` option must be set to
@@ -271,6 +267,24 @@ ffmpeg_branding = "Chrome"
 proprietary_codecs = true
 ```
 
+#### Crash stack unwinding
+
+By default, builds using `is_official_build = true` exclude unwind tables from
+the binaries, as they significantly increase binary size. Google's builds rely
+on crashes being reported using Crashpad, which can then be decoded server-side.
+If you don't intend to enable Crashpad and set up dedicated crash reporting
+infrastructure for your WebView, you may wish to re-enable the unwind tables
+with the following GN argument:
+
+``` gn
+exclude_unwind_tables = false
+```
+
+This will allow Android's debuggerd to produce meaningful stack traces for
+crashes that occur inside WebView's native code. We don't recommend using this
+setting in shipping builds due to the binary size impact, but it may be the only
+alternative if using Crashpad is impractical.
+
 #### Other build options
 
 Other build options may be used but are not supported by the WebView team and

build/config/compiler/compiler.gni

@@ -85,19 +85,21 @@ declare_args() {
   # is_component_build=false. Very large debug symbols can have unwanted side
   # effects so this is enforced by default for chromium.
   forbid_non_component_debug_builds = build_with_chromium
+
+  # Exclude unwind tables by default for official builds as unwinding can be
+  # done from stack dumps produced by Crashpad at a later time "offline" in the
+  # crash server. Since this increases binary size, we don't recommend including
+  # them in shipping builds.
+  # For unofficial (e.g. development) builds and non-Chrome branded (e.g. Cronet
+  # which doesn't use Crashpad, crbug.com/479283) builds it's useful to be able
+  # to unwind at runtime.
+  exclude_unwind_tables =
+      is_official_build || (is_chromecast && !is_cast_desktop_build &&
+                            !is_debug && !cast_is_debug && !is_fuchsia)
 }
 
 assert(!is_cfi || use_thin_lto, "CFI requires ThinLTO")
 
-# Exclude unwind tables for official builds as unwinding can be done from stack
-# dumps produced by Crashpad at a later time "offline" in the crash server.
-# For unofficial (e.g. development) builds and non-Chrome branded (e.g. Cronet
-# which doesn't use Crashpad, crbug.com/479283) builds it's useful to be able
-# to unwind at runtime.
-exclude_unwind_tables =
-    is_official_build || (is_chromecast && !is_cast_desktop_build &&
-                          !is_debug && !cast_is_debug && !is_fuchsia)
-
 # If true, optimize for size. Does not affect windows builds.
 # Linux & Mac favor speed over size.
 # TODO(brettw) it's weird that Mac and desktop Linux are different. We should