First occurrence at 2018-12-04 20:37:32 UTC. Stack trace isn't related to Feed code.

https://ci.chromium.org/p/chromium/builders/luci.chromium.try/android-kitkat-arm-rel/139886

https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8928041623401812768/+/steps/chrome_public_test_apk_on_Android_device_Nexus_5__with_patch_/0/logs/org.chromium.chrome.browser.feed.FeedAppLifecycleTest_testResumeTriggersSchedulerForegrounded/0

java.lang.NullPointerException
	at org.chromium.chrome.browser.toolbar.top.ToolbarPhone.setTabCountProvider(ToolbarPhone.java:2185)
	at org.chromium.chrome.browser.toolbar.ToolbarManager$$Lambda$0.onResult(ToolbarManager.java)
	at org.chromium.ui.AsyncViewProvider$$Lambda$0.onResult(AsyncViewProvider.java)
	at org.chromium.ui.AsyncViewStub.callListeners$1b03825b(AsyncViewStub.java:109)
	at org.chromium.ui.AsyncViewStub.onInflateFinished$1b03825b(AsyncViewStub.java:77)
	at android.support.v4.view.AsyncLayoutInflater$1.handleMessage(AsyncLayoutInflater.java:102)
	at android.os.Handler.dispatchMessage(Handler.java:98)
	at android.os.Looper.loop(Looper.java:136)
	at android.app.ActivityThread.main(ActivityThread.java:5001)
	at java.lang.reflect.Method.invokeNative(Method.java)
	at java.lang.reflect.Method.invoke(Method.java:515)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
	at dalvik.system.NativeStart.main(NativeStart.java)

Reading Android symbols from: /b/swarming/w/ir
Searching for Chrome symbols from within: /b/swarming/w/ir/out/Release/lib.unstripped:/b/swarming/w/ir/out/Release
Searching for native crashes in: /b/swarming/w/itf1jOJV/tmpN_Aizm
Unknown Android release, consider passing --packed-lib.
Searching for Chrome symbols from within: /b/swarming/w/ir/out/Release/lib.unstripped:/b/swarming/w/ir/out/Release
java.lang.NullPointerException
	at org.chromium.chrome.browser.toolbar.top.ToolbarPhone.setTabCountProvider(ToolbarPhone.java:2185)
	at org.chromium.chrome.browser.toolbar.ToolbarManager$$Lambda$0.onResult(ToolbarManager.java)
	at org.chromium.ui.AsyncViewProvider$$Lambda$0.onResult(AsyncViewProvider.java)
	at org.chromium.ui.AsyncViewStub.callListeners$1b03825b(AsyncViewStub.java:109)
	at org.chromium.ui.AsyncViewStub.onInflateFinished$1b03825b(AsyncViewStub.java:77)
	at android.support.v4.view.AsyncLayoutInflater$1.handleMessage(AsyncLayoutInflater.java:102)
	at android.os.Handler.dispatchMessage(Handler.java:98)
	at android.os.Looper.loop(Looper.java:136)
	at android.app.ActivityThread.main(ActivityThread.java:5001)
	at java.lang.reflect.Method.invokeNative(Method.java)
	at java.lang.reflect.Method.invoke(Method.java:515)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
	at dalvik.system.NativeStart.main(NativeStart.java)

Reading Android symbols from: /b/swarming/w/ir
Searching for Chrome symbols from within: /b/swarming/w/ir/out/Release/lib.unstripped:/b/swarming/w/ir/out/Release
Searching for native crashes in: /b/swarming/w/itf1jOJV/tmpzZRhz5
Unknown Android release, consider passing --packed-lib.
Searching for Chrome symbols from within: /b/swarming/w/ir/out/Release/lib.unstripped:/b/swarming/w/ir/out/Release
java.lang.NullPointerException
	at org.chromium.chrome.browser.toolbar.top.ToolbarPhone.setTabCountProvider(ToolbarPhone.java:2185)
	at org.chromium.chrome.browser.toolbar.ToolbarManager$$Lambda$0.onResult(ToolbarManager.java)
	at org.chromium.ui.AsyncViewProvider$$Lambda$0.onResult(AsyncViewProvider.java)
	at org.chromium.ui.AsyncViewStub.callListeners$1b03825b(AsyncViewStub.java:109)
	at org.chromium.ui.AsyncViewStub.onInflateFinished$1b03825b(AsyncViewStub.java:77)
	at android.support.v4.view.AsyncLayoutInflater$1.handleMessage(AsyncLayoutInflater.java:102)
	at android.os.Handler.dispatchMessage(Handler.java:98)
	at android.os.Looper.loop(Looper.java:136)
	at android.app.ActivityThread.main(ActivityThread.java:5001)
	at java.lang.reflect.Method.invokeNative(Method.java)
	at java.lang.reflect.Method.invoke(Method.java:515)
	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:785)
	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:601)
	at dalvik.system.NativeStart.main(NativeStart.java)

Reading Android symbols from: /b/swarming/w/ir
Searching for Chrome symbols from within: /b/swarming/w/ir/out/Release/lib.unstripped:/b/swarming/w/ir/out/Release
Searching for native crashes in: /b/swarming/w/itf1jOJV/tmpmr7EWQ
Unknown Android release, consider passing --packed-lib.
Searching for Chrome symbols from within: /b/swarming/w/ir/out/Release/lib.unstripped:/b/swarming/w/ir/out/Release

Seems like this might be as a result of https://chromium-review.googlesource.com/c/chromium/src/+/1358914/5/chrome/android/java/src/org/chromium/chrome/browser/toolbar/top/ToolbarPhone.java#2182

2018-12-04 20:37:32 UTC is ~12/4 noon west coast time. That above CL merged at 12/4 11:35am. It introduced ToolbarPhone.java:2185

mToggleTabStackButton.setTabCountProvider(tabCountProvider);