Bug 1784208 Part 3 - Remove checking mIsBalancing in FindBestBalanceBSize(). r=emilio

According to our code coverage, the `if` statement is not reachable via any
tests on our CI.
https://coverage.moz.tools/#view=file&revision=7169b8faa7e112c9c4323753434324b93270fc3d&path=layout%2Fgeneric%2FnsColumnSetFrame.cpp&line=1129

After part 1 & 2, `ReflowColumns` can no longer change
`ReflowConfig::mIsBalancing` from true to false since the `ReflowConfig`
argument is now a `const`. Thus, `mIsBalancing` can never be false in
FindBestBalanceBSize().

Differential Revision: https://phabricator.services.mozilla.com/D154429

layout/generic/nsColumnSetFrame.cpp

@@ -945,6 +945,9 @@ void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
                                             ReflowOutput& aDesiredSize,
                                             bool aUnboundedLastColumn,
                                             nsReflowStatus& aStatus) {
+  MOZ_ASSERT(aConfig.mIsBalancing,
+             "Why are we here if we are not balancing columns?");
+
   const nscoord availableContentBSize = aReflowInput.AvailableBSize();
 
   // Termination of the algorithm below is guaranteed because
@@ -1097,16 +1100,9 @@ void nsColumnSetFrame::FindBestBalanceBSize(const ReflowInput& aReflowInput,
     if (!foundFeasibleBSizeCloserToBest && aColData.mFeasible) {
       foundFeasibleBSizeCloserToBest = true;
     }
-
-    if (!aConfig.mIsBalancing) {
-      // Looks like we had excess block-size when balancing, so we gave up on
-      // trying to balance.
-      break;
-    }
   }
 
-  if (aConfig.mIsBalancing && !aColData.mFeasible &&
-      !aPresContext->HasPendingInterrupt()) {
+  if (!aColData.mFeasible && !aPresContext->HasPendingInterrupt()) {
     // We need to reflow one more time at the feasible block-size to
     // get a valid layout.
     if (aConfig.mKnownInfeasibleBSize >= availableContentBSize) {