RFR: 8201647: Make initial clearing of CHeapBitMap optional

[Kim Barrett]

> On Apr 17, 2018, at 8:38 AM, Per Liden <per.liden at oracle.com> wrote:
> 
> CHeapBitMap objects will by default clear the underlying bitmap during construction. In ZGC we don't want mark bitmaps to be cleared on construction, since we instead do this lazily during concurrent marking.
> 
> This patch adds a "bool clear = true" argument to CHeapBitMap's constructor to be able to opt-out of this behavior.
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8201647
> Webrev: http://cr.openjdk.java.net/~pliden/8201647/webrev.0
> 
> /Per

Looks good.