Reland "[content] Set up experiment for smaller memory limits on subframes."

This reverts commit 835df68e168cd201da216e76a9fcff4086d814fa.

Reason for revert: This is a reland of the original CL. This fixes an error with the original that caused component builds to fail.

Original change's description:
> Revert "[content] Set up experiment for smaller memory limits on subframes."
>
> This reverts commit f889f6d17220c5eefbe9171ccb901aee0cda67e6.
>
> Reason for revert: Flaky on multiple Debug bots:
> https://bugs.chromium.org/p/chromium/issues/detail?id=1345150
>
> Original change's description:
> > [content] Set up experiment for smaller memory limits on subframes.
> >
> > This CL sets up an experiment to give the PartitionAlloc thread cache a
> > lower limit in renderers that are only hosting subframes.
> >
> > Everything in this CL is behind a feature flag, so it should not cause
> > any visible changes.
> >
> > Bug: 1331368
> > Change-Id: If912cc8106ebcb791a385b0ba6f245e1c82a7b2e
> > Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3526557
> > Reviewed-by: Benoit Lize <[email protected]>
> > Reviewed-by: Kinuko Yasuda <[email protected]>
> > Reviewed-by: Kevin McNee <[email protected]>
> > Commit-Queue: Thiabaud Engelbrecht <[email protected]>
> > Cr-Commit-Position: refs/heads/main@{#1024797}
>
> Bug: 1331368
> Change-Id: I4d6dc61a4b876643ebb8f0ab973cb3b18df277f6
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3769348
> Bot-Commit: Rubber Stamper <[email protected]>
> Commit-Queue: Trent Apted <[email protected]>
> Owners-Override: Trent Apted <[email protected]>
> Cr-Commit-Position: refs/heads/main@{#1025136}

Bug: 1331368
Change-Id: I2e4f4dac3c795a4f8371988c263df2f82cbb4fcd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3791609
Reviewed-by: Kinuko Yasuda <[email protected]>
Reviewed-by: Benoit Lize <[email protected]>
Commit-Queue: Thiabaud Engelbrecht <[email protected]>
Reviewed-by: Kevin McNee <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1033274}
diff --git a/content/shell/common/main_frame_counter_test.mojom b/content/shell/common/main_frame_counter_test.mojom
new file mode 100644
index 0000000..683ff22
--- /dev/null
+++ b/content/shell/common/main_frame_counter_test.mojom
@@ -0,0 +1,13 @@
+// Copyright 2022 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+module content.mojom;
+
+// This interface is only for the sake of browser test to query the number of
+// main frames in a remote child process.
+interface MainFrameCounterTest {
+    // Returns whether or not the renderer this is called on has a local main
+    // frame.
+    HasMainFrame() => (bool has_main_frame);
+};