crashpad_stackwalker: Specify Android platform when generating symbols

generate_breakpad_symbols.py uses sys.platform as the default platform
if one isn't specified. For me, using crashpad_stackwalker is now
assuming Linux, and failing to generate symbols. Since
crashpad_stackwalker is an Android tool, we can be explicit about what
platform symbols are being generated for.

Bug: None
Change-Id: Iead76cd68b7ffff2b6d4f05d919fa2974687c6e6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1730581Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#683579}

build/android/stacktrace/crashpad_stackwalker.py

@@ -46,6 +46,8 @@ def _CreateSymbolsDir(build_path, dynamic_library_names):
         build_path,
         '--binary',
         unstripped_library_path,
+        '--platform',
+        'android',
     ]
     return_code = subprocess.call(cmd)
     if return_code != 0: