Downsample crash reports on UNKNOWN WebView channel

In case AndroidWebview.apk (the nonchannel build) is still being used by
anyprojects with the assumption that it is the stable/production build,
we should use the same crash reporting sampling rate as we do for
stable/production, so as not to bias the sample of crash reports.

Bug: 963643
Test: Visit chrome://crash in webview shell and check that
Test: GetCrashDumpPercentage is returning the correct value.
Change-Id: If066b3cf1500b0ada2ae9edd8f2944149538b71c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1614579
Commit-Queue: Drew Stonebraker <stonebraker@chromium.org>
Reviewed-by: Richard Coles <torne@chromium.org>
Reviewed-by: Nate Fischer <ntfschr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660574}

android_webview/common/crash_reporter/aw_crash_reporter_client.cc

@@ -78,8 +78,13 @@ class AwCrashReporterClient : public crash_reporter::CrashReporterClient {
       return 100;
     }
 
-    if (version_info::android::GetChannel() == version_info::Channel::STABLE)
+    version_info::Channel channel = version_info::android::GetChannel();
+    // Downsample unknown channel as a precaution in case it ends up being
+    // shipped.
+    if (channel == version_info::Channel::STABLE ||
+        channel == version_info::Channel::UNKNOWN) {
       return kCrashDumpPercentageForStable;
+    }
 
     return 100;
   }