Prevent stripping of C interfaces in framework

We need to redaclare certain interfaces in the GN build files for the
macOS and iOS framework targets to prevent those symbols being stripped
by the linker.

BUG=webrtc:6434

Review-Url: https://codereview.webrtc.org/2431603002
Cr-Commit-Position: refs/heads/master@{#14691}

webrtc/sdk/BUILD.gn

@@ -252,6 +252,7 @@ if (is_ios || (is_mac && mac_deployment_target == "10.7")) {
     "objc/Framework/Headers/WebRTC/UIDevice+RTCDevice.h",
     "objc/Framework/Headers/WebRTC/WebRTC.h",
   ]
+
   if (is_mac) {
     mac_framework_bundle("rtc_sdk_framework_objc") {
       info_plist = "objc/Framework/Info.plist"
@@ -279,6 +280,8 @@ if (is_ios || (is_mac && mac_deployment_target == "10.7")) {
         "VideoToolbox.framework",
       ]
 
+      ldflags = [ "-all_load" ]
+
       configs += [
         "..:common_objc",
         "//build/config/compiler:enable_arc",