[elements] Enable left-trimming again

Essentially a revert of https://codereview.chromium.org/1346013005 but
preserving the refactorings in elements.cc which happened in the mean time.

drive-by-fix: pass isolate as argument to more functions in elements.cc.

BUG=v8:4606
LOG=y

Review URL: https://codereview.chromium.org/1543563002

Cr-Commit-Position: refs/heads/master@{#33023}

src/flag-definitions.h

@@ -195,6 +195,7 @@ DEFINE_NEG_IMPLICATION(harmony, promise_extra)
 
 // Activate on ClusterFuzz.
 DEFINE_IMPLICATION(es_staging, harmony_regexp_lookbehind)
+DEFINE_IMPLICATION(es_staging, move_object_start)
 
 // Features that are still work in progress (behind individual flags).
 #define HARMONY_INPROGRESS(V)                                         \