RFR: 8302849: SurfaceManager might expose partially constructed object

[Per Minborg]

This PR proposes to add `volatile` to a variable declaration in order to prevent partially initialized objects to be observed by threads.

`SurfaceManager` is using a lazily initialized cacheMap that is initialized using a double-checked locking mechanism. Generally, objects initialized using such constructs need to be declared volatile. 

See https://en.wikipedia.org/wiki/Double-checked_locking#Usage_in_Java

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Commit messages:
 - Prevent partially constructed object to be observed

Changes: https://git.openjdk.org/jdk/pull/12654/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12654&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302849
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12654.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12654/head:pull/12654

PR: https://git.openjdk.org/jdk/pull/12654