DevTools: pinned expressions in console

Part 1: introduce pins behind an experiment.
- Prompt has 'pin' button to add current expression to pins
- Pin pane sticks to the top of the Console, with max height
- Pins have multiline editors and a value preview that shows
  "not available" when there is an error

Screenshot: https://imgur.com/a/Ahdtosa

Bug: 849875
Change-Id: Ib0b8bafd97551f823ff8b64d13c5259f42328994
Reviewed-on: https://chromium-review.googlesource.com/1087509
Commit-Queue: Erik Luo <[email protected]>
Reviewed-by: Dmitry Gozman <[email protected]>
Reviewed-by: Joel Einbinder <[email protected]>
Cr-Original-Commit-Position: refs/heads/master@{#574740}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 589cdcb33ef96770a2c3e4a9c76ee439d1e2e56e
diff --git a/front_end/main/Main.js b/front_end/main/Main.js
index 5ac2f23..c275c35 100644
--- a/front_end/main/Main.js
+++ b/front_end/main/Main.js
@@ -114,6 +114,7 @@
     Runtime.experiments.register('nativeHeapProfiler', 'Native memory sampling heap profiler', true);
     Runtime.experiments.register('networkSearch', 'Network search');
     Runtime.experiments.register('oopifInlineDOM', 'OOPIF: inline DOM ', true);
+    Runtime.experiments.register('pinnedExpressions', 'Pinned expressions in Console', true);
     Runtime.experiments.register('protocolMonitor', 'Protocol Monitor');
     Runtime.experiments.register('sourceDiff', 'Source diff');
     Runtime.experiments.register('sourcesPrettyPrint', 'Automatically pretty print in the Sources Panel');