[tools/perf] Disable browse:chrome:omnibox separately on WebView and N5

To disable on multiple platforms, each disable must be listed
separately as a new entry.

Also update docs to make this a bit more explicit.

TBR=sullivan@chromium.org,nednguyen@google.com
NOTRY=true

Bug: 819552
Change-Id: I0f033a9ea11d8df6a27fcbbfaa5cc98d408ac2f1
Reviewed-on: https://chromium-review.googlesource.com/955682Reviewed-by: Juan Antonio Navarro Pérez <perezju@chromium.org>
Commit-Queue: Juan Antonio Navarro Pérez <perezju@chromium.org>
Cr-Commit-Position: refs/heads/master@{#541787}

docs/speed/perf_bot_sheriffing.md

@@ -351,9 +351,11 @@ linked to multiple bugs, an entry for each bug is needed.
 
 A list of supported conditions can be found [here](https://cs.chromium.org/chromium/src/third_party/catapult/telemetry/telemetry/story/expectations.py).
 
-Multiple conditions when listed in a single expectation are treated as logical
-_AND_ so a platform must meet all conditions to be disabled. Each failing
-platform requires its own expectations entry.
+When multiple conditions are listed in a single expectation they are treated
+as a logical _AND_, so the platform must meet _all_ conditions to be disabled.
+
+If you need to disable a single story on multiple different platforms, you must
+list each platform separately on its own line as a separate entry.
 
 To determine which stories are failing in a given run, go to the buildbot page
 for that run and search for `Unexpected failures` in the failing test entry.

tools/perf/expectations.config

@@ -250,7 +250,8 @@ crbug.com/812807 [ Linux ] system_health.memory_desktop/browse:media:tumblr [ Sk
 crbug.com/787001 [ Android_Webview ] system_health.memory_mobile/load:media:soundcloud [ Skip ]
 [ Android_Webview ] system_health.memory_mobile/browse:chrome:newtab [ Skip ]
 crbug.com/714650 [ Android ] system_health.memory_mobile/browse:news:globo [ Skip ]
-crbug.com/819552 [ Android_Webview Nexus_5X ] system_health.memory_mobile/browse:chrome:omnibox [ Skip ]
+[ Android_Webview ] system_health.memory_mobile/browse:chrome:omnibox [ Skip ]
+crbug.com/819552 [ Nexus_5X ] system_health.memory_mobile/browse:chrome:omnibox [ Skip ]
 crbug.com/657433 [ Android ] system_health.memory_mobile/load:tools:gmail [ Skip ]
 crbug.com/728081 [ Android ] system_health.memory_mobile/browse:news:toi [ Skip ]
 crbug.com/777355 [ Android ] system_health.memory_mobile/long_running:tools:gmail-background [ Skip ]