RFR: 8316305: Initial buffer size of StackWalker is too small caused by JDK-8285447

[Mandy Chung]

A trivial fix.   [JDK-8285447](https://bugs.openjdk.org/browse/JDK-8285447) intends to change the initial batch size only for a stack walker with an estimated stack depth. For stack walkers without user-supplied estimated stack depth, the initial batch size is changed to 3 which is a bug.  This causes the stack walker to fetch the second batch after walking 2 frames.

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Commit messages:
 - 8316305: Initial buffer size of StackWalker is too small caused by JDK-8285447

Changes: https://git.openjdk.org/jdk/pull/15749/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15749&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8316305
  Stats: 8 lines in 1 file changed: 0 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/15749.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15749/head:pull/15749

PR: https://git.openjdk.org/jdk/pull/15749