Make RTC_OBJC_EXPORT respect is_component_build.

While RTC_EXPORT is aware of component builds (selecting "default"
visibility only when WebRTC is built as a shared library),
RTC_OBJC_EXPORT (which predates RTC_EXPORT) was always marking symbols
as "default" visible.

This CL fixes the problem but on the other hand it will require
standalone builds of the WebRTC.framework to set the GN argument
`rtc_enable_symbol_export` to true.

No-Presubmit: True
Bug: chromium:1159620
Change-Id: I4a16f9bd3c1564140a5a30f03b3e77caed1df591
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/198082
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Henrik Andreassson <henrika@webrtc.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#32856}

tools_webrtc/ios/build_ios_libs.py

@@ -146,6 +146,7 @@ def BuildWebRTC(output_dir, target_arch, flavor, gn_target_name,
     gn_args.append('enable_ios_bitcode=' +
                    ('true' if use_bitcode else 'false'))
     gn_args.append('use_goma=' + ('true' if use_goma else 'false'))
+    gn_args.append('rtc_enable_symbol_export=true')
 
     args_string = ' '.join(gn_args + extra_gn_args)
     logging.info('Building WebRTC with args: %s', args_string)