Quota: Avoid integer overflow.

In the storage pressure check, we multiply available space by 100.
The max value of an int64_t is 2^63, so an overflow could happen on
a disk whose size is at least 2^63 / 100 ~= 92 PB. This change
refactors the storage pressure check by representing the storage
pressure threshold (2%) as 0.02 rather than 2, which allows us to
remove the multiplication by 100 on the other side of the equation.

Bug: 1127237
Change-Id: If3d5a89ee2cd8ea8f0beabf1a38ae71a14d3ff52
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2429365
Reviewed-by: Marijn Kruisselbrink <mek@chromium.org>
Commit-Queue: Jarryd Goodman <jarrydg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810769}

storage/browser/quota/quota_manager.cc

@@ -53,7 +53,7 @@ constexpr int64_t kReportHistogramInterval = 60 * 60 * 1000;  // 1 hour
 
 // Take action on write errors if there is <= 2% disk space
 // available.
-constexpr double kStoragePressureThresholdPercent = 2;
+constexpr double kStoragePressureThresholdRatio = 0.02;
 
 // Limit how frequently QuotaManager polls for free disk space when
 // only using that information to identify storage pressure.
@@ -1487,7 +1487,7 @@ void QuotaManager::MaybeRunStoragePressureCallback(const url::Origin& origin,
     return;
   }
 
-  if (100 * available_space < kStoragePressureThresholdPercent * total_space) {
+  if (available_space < kStoragePressureThresholdRatio * total_space) {
     storage_pressure_callback_.Run(std::move(origin));
   }
 }