Mac triage: marking assigned for erikchen@

This is not related to keyboard changes. 

MessagePumpMac requires that the NSApplication conform to CrAppProtocol. See https://cs.chromium.org/chromium/src/base/message_loop/message_pump_mac.h?l=51. 

"""
[74960:775:0619/164140.506505:558128056788279:FATAL:message_pump_mac.mm(909)] Check failed: [NSApp conformsToProtocol:@protocol(CrAppProtocol)]. 
0   libbase.dylib                       0x0000000113d0cf6e base::debug::StackTrace::StackTrace(unsigned long) + 174
1   libbase.dylib                       0x0000000113d0d02d base::debug::StackTrace::StackTrace(unsigned long) + 29
2   libbase.dylib                       0x0000000113933a2c base::debug::StackTrace::StackTrace() + 28
3   libbase.dylib                       0x00000001139b2b4c logging::LogMessage::~LogMessage() + 460
4   libbase.dylib                       0x00000001139b08b5 logging::LogMessage::~LogMessage() + 21
5   libbase.dylib                       0x0000000113a1b824 base::MessagePumpMac::IsHandlingSendEvent() + 532
6   libcontent.dylib                    0x000000012393d361 content::WebContentsViewMac::IsEventTracking() const + 17
7   libcontent.dylib                    0x00000001238f4ad6 content::WebContentsImpl::Close(content::RenderViewHost*) + 70
8   libcontent.dylib                    0x00000001233c10d7 content::RenderViewHostImpl::ClosePageIgnoringUnloadEvents() + 87
9   libcontent.dylib                    0x00000001233c3085 content::RenderViewHostImpl::OnClose() + 21
"""

We also get some nice warning messages on a local component build:
"""
objc[74961]: Class MockCrApp is implemented in both /Users/erikchen/projects/chromium/src/out/gn/libtest_runner.dylib (0x1407110d0) and /Users/erikchen/projects/chromium/src/out/gn/App Shell.app/Contents/Frameworks/App Shell Framework.framework/Versions/A/App Shell Framework (0x107e8b890). One of the two will be used. Which one is undefined.
"""

And the ToTMac builder also uses component builds:
https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.clang%2FToTMac%2F1760%2F%2B%2Frecipes%2Fsteps%2Fgenerate_build_files%2F0%2Fstdout
"""
clang_use_chrome_plugins = false
is_clang = true
is_component_build = true
is_debug = false
llvm_force_head_revision = true
symbol_level = 1
"""

This would explain why we don't see the error on any other builders. Looks like the relevant CL is:

https://chromium-review.googlesource.com/1083093
"""mac: Make extension_browsertests work in component builds."""

Possibly a dupe in 853172

of, too