[AW] Add a comment to why default channel crashes are sampled out

This comment was dropped when moving crash reports sampling from crashpad to upload service. It provides more context why we sample crashes for default channel (debug/unknown channel).

Bug: N/A
Change-Id: I1d949aafdd59541b9f61fdc23885481f46c16322
Test: N/A
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2075117
Commit-Queue: Hazem Ashmawy <hazems@chromium.org>
Commit-Queue: Tobias Sargeant <tobiasjs@chromium.org>
Reviewed-by: Tobias Sargeant <tobiasjs@chromium.org>
Auto-Submit: Hazem Ashmawy <hazems@chromium.org>
Cr-Commit-Position: refs/heads/master@{#744682}

android_webview/nonembedded/java/src/org/chromium/android_webview/services/AwMinidumpUploaderDelegate.java

@@ -83,6 +83,7 @@ public class AwMinidumpUploaderDelegate implements MinidumpUploaderDelegate {
         return new CrashReportingPermissionManager() {
             @Override
             public boolean isClientInMetricsSample() {
+                // Downsample unknown channel as a precaution in case it ends up being shipped.
                 if (mSamplingDelegate.getChannel() == Channel.STABLE
                         || mSamplingDelegate.getChannel() == Channel.DEFAULT) {
                     return mSamplingDelegate.getRandomSample() < CRASH_DUMP_PERCENTAGE_FOR_STABLE;