Explore removing/disabling chunk separator feature

[niloc132]

IframeLinker (deprecated) and CrossSiteIframeLinker (the current default) have a feature to split up strings passed to script blocks. There are roughly two considerations that have historically mattered here:

• If chunks are too small, there are too many <script> blocks, which breaks IE (see defunct class JsIEBlockSizeVisitor, removed in github.com/gwtproject/gwt/commit/11bcab6517cb27940bb00df26f2be2310c73150b)
• If chunks are too big, performance is bad in loading large <script> blocks in Firefox

The firefox performance issue was noted in 2009, and may no longer apply. Additionally, only the initial fragment has this applied anyway - all split points appear to have their output un-chunked (suggesting that these fragments loaded "slowly" in firefox at the time).

The impact of this feature is low - it helped contribute to #9931, and will require 2-3x the memory of the primary fragment in order to copy it. No user code can depend on this as a feature - it if there is no performance benefit to keep it, it seems easily worth removing/disabling.

It seems likely that this feature no longer matters, If so, we should consider one or more of the following:

• Set the iframe.linker.script.chunk.size configuration property to -1 to disable the feature - this is a supported value, but the default is presently 30000.
• Detect non--1 values when sourcemaps are enabled, optionally with a warning if another value is set.
• Deprecate/remove the feature entirely.

See also:

• 7bcb57a
• 4814ffa