Description:
- After deserialization we notify the GC to trim the pages of the snapshot, i.e. to get rid of unused memory. 
- We do that using the high water mark on a page
- In debug mode we even do a bit more: We check that when we iterate the page, after the last object there is a filler that corresponds to the high water mark.

The problem is that the deserializer sometimes (test case) creates a filler that it doesn't use. If this happens to be the last filler, then the GC thinks the high water mark is off.

Repro:
Commit: 4ed27fc836acfc3218a5e4ce6d878a513e9df788
PS: http://crrev.com/2307903002#ps120002

gn args:
is_component_build = false
is_debug = false
use_goma = true
target_cpu = "x86"
dcheck_always_on = true
symbol_level = 1

out/Release/cctest --random-seed=-564235616 --ignition-staging --turbo test-serialize/SnapshotCreatorMultipleContexts --nohard-abort --nodead-code-elimination --nofold-constants

It might very well be that the deserializer is allowed to leave fillers behind, in that case please just close the bug.