[privacy_budget] Increase MaxMax to 40.

kMaxIdentifiabilityStudyMaxSurfaces places a hard-coded maximum for the
number of surfaces that can be reported by a client regardless of server
configuration.

After running the study on pre-release channels we've determined that
the existing hardcoded limit can be increased without affecting our
privacy guarantees.

The existing limit of 10 was based on the initial assumption that the
number of surfaces would be in the order of ~60. The actual number is
~1000. Hence the increase.

R=pauljensen@chromium.org

Bug: 973801
Change-Id: Iff5f7a10db6180836d63519c59074ea2a61e6710
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2629527Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Commit-Queue: Asanka Herath <asanka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#843539}

chrome/common/privacy_budget/privacy_budget_features.h

@@ -85,7 +85,7 @@ extern const base::FeatureParam<int> kIdentifiabilityStudyMaxSurfaces;
 // In other words this is the maximum value that can be configured via
 // `kIdentifiabilityStudyMaxSurfaces`. Hence it's the
 // `kMaxIdentifiabilityStudyMaxSurfaces`.
-constexpr int kMaxIdentifiabilityStudyMaxSurfaces = 10;
+constexpr int kMaxIdentifiabilityStudyMaxSurfaces = 40;
 
 // Selection rate for clusters of related surfaces.
 //